LloydPDX

You're right about the additional powering solution. Fixed on a tripod indoors for something like a livestream or studio shoot, it's no problem to run it off mains power with the included power adaptor. For mobile shoots, you can turn that particular lemon into lemonade by using a V-mount battery or an NP-F battery with USB and DC outlets to also power you camera (with a dummy battery or a USB-C cable) and even another accessory. I have one rig built up with mini-cine style with a V-mount and two of the super cheap Powerextra 48 Wh batteries for my more minimalistic setup and they work great. 
The balance can be an issue with a heavy battery up high if you go the NP-F route. I personally tend to mount the monitor off to the left on that rig either on top of a side handle or just using a magic arm as a field-expedient side handle off the nato rail built into the side of my cage. 
If you go with a side handle or just on top of the camera/cage/top handle, do yourself a favor and use a monitor mount like Smallrig's that you can tighten with an allen key. Regular ball heads will always come loose. I think I'm going to try one of the Nitzi mounts because they tighten with an allen key AND are the only monitor mounts I've found with Arri-style locating pins. I'm not sure why no one else has them. The monitor is built for them and why wouldn't you want to it in so it won't twist when you are trying to swivel it?! Even the ballhead I got from Atomos kit doesn't have them for crying out loud. 
Anyway, probably deeper into the weeds than you were interested in. But there you go.

I shoot Raw for stills @LloydPDX ...to date anyway, but looking at all options now I have moved to Panasonic.

I capture Raw almost exclusively for all my still work, but I have to remember that good event photographers (and others) might have their sRGB jpgs dialed in so well that their workflow is simplified. And it looks good! Maybe time for an old dog to learn new tricks.

This has been a really helpful thread with thoughtful responses. Thank you. For now, I’ll be pushing myself with the D850 for a better real-world understanding of its capabilities. And maybe borrowing/renting another cam to feel the difference in similar situations, and in the edit.

Yes, this, today, would be the most likely direction. But I’m going to push myself a bit more with the D850 in real life for now. Used Gh5s’ and other choices will be around for awhile, after I have more informative successes and failures.

This response reflects where I'm at right now... asking myself if for now, i really need 10 bit, how much can i do with 8bit and existing profiles (and I have Andrew’s z-log profiles to experiment with further.) Chromakeying is still a question, but some focused trials will shed more light on workability. I can also rent a Ninja recorder to get me to 422  in that specific case.
Your observation about Nikon’s history of r&d was helpful reframing!

🙂
Good plan.  I find that by using what you have and then seeing what doesn't work for you in real-world shooting, doing the projects that you do, in the way that you do them, with your own particular expectations and tastes, then you'll end up spending the money on what actually matters to you.
There's lots of times when something is needed by someone else, but that doesn't mean you should upgrade.  There's also lots of people recommending extra features "just in case" which can create a kind of spec inflation that's not based on reality, especially when people read the just-in-case recommendations and then pass them on to others with their own just-in-case inflation added on.

This response reflects where I'm at right now... asking myself if for now, i really need 10 bit, how much can i do with 8bit and existing profiles (and I have Andrew’s z-log profiles to experiment with further.) Chromakeying is still a question, but some focused trials will shed more light on workability. I can also rent a Ninja recorder to get me to 422  in that specific case.
Your observation about Nikon’s history of r&d was helpful reframing!

This has been a really helpful thread with thoughtful responses. Thank you. For now, I’ll be pushing myself with the D850 for a better real-world understanding of its capabilities. And maybe borrowing/renting another cam to feel the difference in similar situations, and in the edit.

Well... the two things that stood out to me in your above elaboration are overall size and 10-bit color, which to me are totally legitimate concerns. Apologies if I am placing too much emphasis on those points. Also I know you had mentioned previously that you didn't want to blow a hole in your budget.
So I guess if it were me I would be looking pretty strongly at the X-T3 and the G9 - used / refurbished if possible. Stretching for the GH5 would be tempting since around here they go for about $1,000 used with the V-LOG update. I don't know about adapting Nikon lenses for those cameras though.
Be warned that a camera like the GH5 will have 108 different video parameters that you will want to adjust to your preference. Setting up my Panasonic S1 took a couple of hours, and in the middle of shoots I find myself cursing out loud and wondering why I didn't set parameter X to a custom function button / quick menu / my menu and have to dig in through the main menu to change it.

I agree with what you are saying, but if you are going to be doing client work, then there comes a point where you need to look at the gear and say, "Can I either 1) make more money, or 2) spend more time with my loved ones" with this new gear.

I agree about lock-in, but look at how many thousand/million posts there are online that basically distill down to "hi, two years ago I spent $3000 on a camera body that has 5-bajillion pixels, and now I can't sleep at night until the brand I worship with my all my heart and will fight to the death online to defend releases a camera with 5.2-bajillion pixels that I can spend 150% of what I spent on my current camera body on a new camera body that will give me a 2.4% improvement over the performance of the one I already own".
and if you want to talk about existential crisis', then don't get me started about vintage lenses....  
8-bit vs 10-bit is an interesting topic.  I'm a big fan of 10-bit and I bought my GH5 over other options specifically because it had 10-bit.
However.....  a large (very large) percentage of the time, that difference doesn't matter.
You can shoot in one of two ways.  
The first is to shoot LOG, which is best done by shooting fully manually, using technical methods to expose correctly (eg, grey cards, or even light meters), using a view LUT probably on an external monitor, and then spending significant time in post to colour grade the image to get the absolute most out of it.
The second is to shoot in a 709-style colour space, where you can expose using in-camera tools like waveforms or zebras, and then the time spent in post is minimal, and you're just tweaking the existing colour science that the camera has already given you and you were seeing on-set.
The first method will give you the greatest DR and best results, if you know what you're doing. However, the second method can basically be paraphrased as "Nikon has optimised the exposure tools in your camera to work optimally with the colour science that the Nikon colour scientists spent decades developing and optimising".  
So, the first one will only give you better results if you want something very different to what the Nikon colour scientists predicted you would want, or if you are a better colourist than the Nikon colour scientists.
I swallowed the hype online about shooting with LOG and using LUTs and colour grading in post, and I spent years shooting and creating images that weren't as good as the default colour profile in the cameras I had.  I now shoot in 10-bit, but I use a 709-style picture profile to give me a great starting point, and I adjust from there.
Almost all the arguments online about shooting 10-bit are really about shooting LOG, and more than half of the discussion about shooting LOG is coming from hipster YouTubers who just want to sell you their LUT pack.  
Obviously you're free to do as you choose, and there's more to a new camera than just 8-bit vs 10-bit, but I would highly encourage you to dig a bit deeper into each aspect of the camera, each specification, and to really challenge the idea that you need it or that it will even help you.
Shooting LOG requires colour grading in post that takes considerable time and effort.  Most professional videographers I've seen who talk about productivity and efficiency and keeping their clients happy and *gasp* running a profitable business, rather than endlessly talking online about specifications, use an 8-bit codec and a customised 709-style colour profile, and either don't colour grade at all, or apply a preset look they've developed that just tweaks the image a bit.  Their focus is on getting final videos done and out the door, rather than shooting LOG and arguing about DR on camera forums.

Unfortunately for someone like me who shoots real estate, I have found 10-bit LOG to be really useful 🙂
When you are trying to capture a dark bedroom with a beautiful, sunny view of the Pacific Ocean, that extra dynamic range comes in handy.
And the color casts from mixed lighting are a lot easier to deal with in 10-bit footage. I mean, people are paying stagers $8,000 to $12,000 to have furniture brought in for the photos and videos, and the stagers are putting in bulbs of different color temperatures in the same small room 😞 

I agree that maybe the Ninja V might be an ideal solution instead of buying a new old camera, but if you're set on buying something perhaps the GH5/s with a focal reducer/adapter to use your Nikon glass might be the way to go? 

Ditto except; Classic Chrome profile on the XH1, then Eterna profile on the XT3 and now Natural profile on the S5.
Has there ever been a time I wish I’d shot log? Nope.
OK, it’s the video equivalent of shooting Jpeg vs Raw, but for me and my needs, I’m comfortable with it and all about getting kit and process (ing time) out of the way as much as possible.

Helpful point about stabilizing in post! 
True observations about gear lifetimes, I think. Good mics can last a career, and beyond, as can lenses. My 2008-released Panasonic HPX170, bought used, has great features and taught me lots about video. As problematic as the sensor limitations/ image quality look now, I’m loathe to sell it. But I won’t use it for any client work, so it’s time to move to better image quality with good features, on a used, new-to-me camera. Which I’ll probably squeeze every last pixel out of, too.
Though a Ninja can be very useful, for now compactness and minimal rigging is a priority. And, max quality out of the d850 is still only 8 bit. (Nikon, what???) The increase to 422 can be marginally useful, but seems like a minimal return on investment.
I appreciate your experienced and considered comnents.

My thinking is to add one of the cameras I’m considering as a primary, with my 850 and LX100 as potential b-cams or backups for now, or renting a second cam as needed. This would evolve with time.
I’ve picked up some bargain second hand kino-type lights (Lowel) and have a couple LED ac/dc monolights with modifiers.
Audio of course is significant too. I got into sound-for-picture before video though, and I have some good kit already.
 

I've returned to older cameras. I don't need 4k for personal projects and don't really think it looks better, either.
I've always liked the image from both the C100 and GH5. I think V Log has +4 stops of highlight detail above 18% gray and Canon Log has 5.3 but then the shadows get a bit muddy and you have to watch your 100+ IRE super whites and recover them in post. With an external recorder, the C100's image I think is really great. But it does fall apart bit with macro blocking on leaves and flat surfaces, so I wouldn't use it for green screening necessarily. The Mk II might be better.
But after going through a bunch of newer cameras I also found the older stuff familiar and lighter in post, which I appreciate. 

As a pro Nikon user for 40 years, I have mixed feelings here. I love my d850 for its still image quality and features. But I’d guess their contract with Sony for the sensor limited the video features and codec. As I look to making more video, I only have to look at a used Lumix GH5S for example to see a simple solution for exceptional video features and in-camera 10bit, which for me are more relevant than full frame vs m43. Disappointed by the Z ii offerings. So I won’t invest in a Z body, and will instead likely add a video-centric camera from another maker, even as I still swoon over my 2-year old Nikon 850.

As a pro Nikon user for 40 years, I have mixed feelings here. I love my d850 for its still image quality and features. But I’d guess their contract with Sony for the sensor limited the video features and codec. As I look to making more video, I only have to look at a used Lumix GH5S for example to see a simple solution for exceptional video features and in-camera 10bit, which for me are more relevant than full frame vs m43. Disappointed by the Z ii offerings. So I won’t invest in a Z body, and will instead likely add a video-centric camera from another maker, even as I still swoon over my 2-year old Nikon 850.