GreekBeast

First post in here! 
@GreekBeast do you live in Greece too? Because this could play a role in buying options.
Your choices depend on a number of things: do you absolutely need AF? Will you often use a gimbal? 
1) If you want AF on all lenses, the (Canon) Metabones + Sigma 18-35 is a great choice. There is also the much more economical Viltrox adapter (also for Canon) which has received rave reviews but is backordered almost everywhere. I guess you can expect more than two months to receive it in Greece (due to customs clearance times...)
2) If you go the Nikon way (as I've done) there is the Viltrox NF-43x adapter, which features the same focal reducer as the (AF) Canon version, but is much cheaper, easier to get and of course manual focus only. If you already have a number of Nikon lenses, it surely makes sense to go this way. Again, no AF. 
3) I have the Sigma 30mm f1.4. It's a quite nice lens, sharp and with reasonably fast AF. You can usually get this for close to 200€ used in Greece and about 330€ brand new. Haven't tried the 16mm but I'd think the Panaleica 15mm f1.7 would make a better choice; smaller, lighter, probably faster AF. Great for gimbal or handheld work. 
In conclusion, IMHO, the most cost effective solution would be to go for the Viltrox for Nikon focal reducer, plus Sigma 18-35mm, plus 2-3 native lenses that cover your most used focal ranges (the Panasonic 42.5mm f1.7 would make another good/cheap option). Because of your low light requirements, native zooms would present some issues. Then add a couple of your adapted Nikon glass and you'll be well served. 
 
 

I don't particularly mind him but he definitely lost me from 7:42 with the "2 times better image quality than the X-H1" stuff to be honest.
 

Its interesting how much raging debate there was in threads about these two cameras when they were announced.
Did anyone on here actually ever buy one in the end?

It differs from territory to territory.  i believe the GH4 sold for the EU market has the limitation, other models do not.  I also believe (from memory) that the GH5 is unrestricted in all territories.

I just got my hands on an A7rIII and the greatest drawback of A7rII has been fixed: Shutter sounds great! 

I read somewhere on canonrumours I think where one of their execs basically said this was their first push into what will be a renewed plan to enter into the mirrorless market. So I am expecting more to come, and when the later iterations do come, I think most people will be pleased. They have to cripple this one to distinguish it from their higher end models (if it isnt simply a matter of hardware limitations in a small body). Once canon get it right, they will be leaders yet again - not because they have the best specs like sony, but because they are able to execute a camera correctly. 
Ask yourself this question: which camera would you more readily recommend to a soccer mom - Canon M50 or a Sony a6300? I think any rational person would pick the canon. The a6300 is too complicated a camera for the average person to use between the menus and the ergonomics. I've heard it multiple times from people in museums complaining about how they don't know how to use the camera. If they don't know how to use it they will put it down.    

I am hopping for the following: 

SAME Camera as A7III with same sensor but: 
+ weather sealing and A7RIII screen/viewfinder (this is a given...) 
+ 4k60p
+ 1080-240p
+ 10 bit
Keeping the same A7III sensor is great: Makes it cheaper for Sony, gives us A7III-like image quality for stills (12mp not good enough for me) and most important allows for APSC video mode. Having 2 lenses in one is just great. Put a 20 or 24mm on that and you get a very wide + standard 35mm, and put a light 50mm prime and you get a useful standard 50mm + a portrait lens 75mm. 
This gives you 24/35/50/75mm covered with just 2 lenses. How amazing is that? 

For me, this would blow the GH5. Just make it Sony! 
Oh, and the only potential issue with my spec list would be Rolling Shutter with the 24mpx..... Hopefully they can sort that out. 

Iso for Iso, but it looks almost funny how Yuryev's face suddenly becomes the clearest/sharpest at 1600 in GH5

That awkward moment when A9 owner realizes he spent $2500 more than he should. 

Great, A7S II and A7R II used prices will drop massively now. Its basically an A7S II that can also take good photo's, for almost half the price. If I was a A7S II user I would sell it fast to fund a new A7III.

https://***URL removed***/samples/9120111657/sony-a7-iii-sample-gallery
There is a lot of yellow in these images. Wonder if the other colour profiles will tone it down a bit. Otherwise looks pretty good.

You didn't mention the pros of the speedbooster nor the downsides to going native as if Panasonic paid you.

Here is a hotel I've shot with the Gh5 and the native 12-35 /2.8 ( first gen). It had a lot of ronin so if you shoot Run N Gun you might get a feel. 

@Andrew Reid, I don't know how many times I've recommended for you to ditch Premiere and go FCPX. 
I've been using FCPX since the very first version, and because it was so different, I couldn't even put two clips together. But I persisted. 
To this day, I'd say FCPX has been the greatest creative decision I've ever made. Editing is more enjoyable, it's faster, it's seamless, it's a time saver. 
You have to regard that FCPX is not Premiere and does everything it can not to be like Premiere. It's a totally different machine. You don't try and drive a car like you would a motorcycle! 
When you learn to use it fluidly, your head starts shaking in utter shame of the people who trash it day in, day out. Their loss. 
To me, Premiere feels like a dinosaur with broken legs. It's still editing the "old way". FCPX was an ambition ahead of it's time, and now has matured into what I think is the best editing program today. 
Persist with it, learn it, and I'm certain you will love it. 

Once you get use to doing things the FCPx way, every other NLE I've used - Premiere, Avid and Grass Valley - is just so slow in comparison. The above video Axel posted is one of a few I watched after the 10.3 update that convinced me to spend some time really learning FCPx after working with the others for years. Now I can't go back. Resolve just doesn't jive with me at all either. FCPx is my do-it-all solution. YMMV.
Chris

Talking pure 4K quality alone, the picture seems to have an edge on the D850. Shooting a comparison now, including the GH5S and it's clear. The D850 has less moire and aliasing, more solid and convincing detail, which doesn't have as much false detail on fine patterns. It's a small but significant difference. Codec and colour seem to have an advantage on the D850 as well. I'm surprised. It's like a 'Nikon' 1D C.
I wonder why Sony put all their latest tech into Nikon's 46MP sensor and stuck with the slightly less capable 42MP sensor themselves?
There are other differences though... A7R3 has proper LOG modes not just a flat profile. I've yet to test dynamic range, will do soon.
In low light, I realised I made a mistake in the last test that focussed on the GH5S. The D850 had noise reduction completely off. It really does seem to turn it off and give you a fine noise grain. Need to retest the ISO 12,800 vs A7R3 and see the impact of noise reduction on both cameras, and whether it turns off completely on the Sony (or not) and which has the most capable system of reduction. We all know the A7R3 noise reduction in full frame 4K is pretty much magic. Does the Nikon do a similar job?
If we're talking video features and form factor it's a non-contest... A7R3 cleans up across the board especially with it's superb video AF and stabilisation.