SRV1981

+1 for getting white balance right in camera.  
Sony is known to be harder to get good colour from but that just means that you have to be more particular with WB and processing in post.
Kraig Adams on YouTube is known for getting great results in camera and is currently using the A7iii.  He made a few videos about it that might include his colour profile and settings.

If you can sell directly to a buyer on FM or EOSHD or another online board that has a buy and sell forum, that's your best option for getting the most money. If you're in need of a dealer trade in, you'll take a hit on price (you pay for the service through the low trade in price).

Speaking only about purchasing used and/or refurb...I've had great success (luck) with B&H, Fred Miranda's Buy and Sell board and Lens Authority (best to be price savvy when using all). Haven't heard of MPB until now (thanks, where have I been?!). As for selling or trading in...my daughter has seen to it that I've never had to concern myself on that matter! LOL :D

No, come to the darkside. ?

Just put the a7iii in full cinema mode and it will look awesome straight out of camera. It’s hidden in the menus.

Fair point, D3s has a 50% longer shutter life, but there is also everything else about a heavily worn body over the years than just the shutter life. A cheap D3s has gone through the wars, while cheap D500 might just be a hobbyist who is bored and moving over to mirrorless now. 
 

Would cost an arm and a leg. 

Then after that....  why not just wait for the A7R V?

Then after that, wait for the A7R VI instead?

 And then...

Yeah that makes sense, why not have the best tool for the job. I think as an amateur he'd probably be happy with the A73 though. For something like sports I'd probably rather have something with more of a reach that S35 gives.

I understand the D4 against an X-T2, however the A73 is a different beast. I imagine the AF might be a bit better but besides that how do you think the D4 would come out ahead?

My perspective as a professional sports photographer is if you are going to be shooting stills in that environment a lot then you should have a dedicated camera that is made for the job so I'd be looking at a used Nikon D4 and Nikkor 70-200mm f2.8 which you should be able to pick up for around £1500-1600.
If you want to cut the budget a bit then substitue in a D3s instead of the D4 or up it to a D4s if you have a bit more budget.
I trialled shooting with the X-T2 system as an alternative/supplement to Nikon about 3 years ago and whilst it made decent images it just wasn't up to the job at the same level and the experiment didn't last long.
For video, you don't need a particularly low light monster as you won't need the same shutter speed so your 1000th/f2.8/ISO6400 even if you want to shoot 50p and have 180 degree shutter is only ISO640. Incidentally, the reported exposure from the X-T2 exposure will be innacurate as shooting one alongside a D4 in fixed lit stadiums, the X-T2 would be at ISO6400 and the D4 would be at ISO5000.
The Nikon Z6 would be the obvious choice for the video to keep in the same brand as the tests I've seen with the FTZ adapter and the 70-200mm f2.8 show it performing well.
Again, if you go for used then you should be able to pick up both cameras and the lens for around £3K.
For that you will be getting a genuine professional sports stills camera that will take the shooting conditions you describe in its stride, a very capable 4K video camera with IBIS and a ProResRAW option (any year now) and a fast, stabilised high quality optic.
 

The problem with the A9 for is that the mechanical shutter is capped to 5fps which means that you have to use the electronic shutter and run the very real risk of banding from LED boards in most pro sports arenas and stadiums.
That was one of the factors that seriously hindered the A9 being accepted by its target audience although I did a couple of Champions League matches last week and saw the first two I've ever seen being used pitchside.
Whether that was due to Sony's sponsorship arrangements with UEFA meaning they were being trialled or whether the tide has turned for them remains to be seen.
 

Thanks!  It sounds like the Sony A73 is the best single camera for my needs.  I will look at some video clips and read some reviews before going to try it myself.  I would love to look into the X-T3 but worried about lowlight.  Panasonic S1H seems excellent but AF sounds terrible?

I understand.
I would just kindly suggest that it might be pretty difficult to find anything that is going to be a SIGNIFICANT upgrade over your current X-T2 system for $5K or less.
For STILLS:
the Sony a6400 / a6600 body is going to have the best AF tracking this side of the Sony a9... but I don't know if you will be able to get away with the $550 85mm f/1.8 (which will probably result in LOTS of cropping in post), or whether you will need the $2,600 70-200 f/2.8 GM OSS. 
The 85mm f/1.8 will allow you to lower your ISO by about one stop - maybe more since Fuji cameras tend to expose darker at equivalent ISO values. The f/2.8 of the 70-200 won't allow as much light. The tests I have seen show that the Sony's tend to have cleaner high iso performance than the Fuji cameras, but part of that is due to built-in automatic noise reduction which means that the fujis tend to be SHARPER. But that is a generalization.
The 85mm f/1.8 has no built in stabilization. The 70-200 does. The a6400 ($900) does NOT have stabilization, and doesn't have the AF tracking in VIDEO that the a6600 ($1,400) has. The a6600 also has built in stabilization.
And again, don't know if the AF performance of the 85mm f/18 will keep up with the advanced tracking of the Sony a6400 / a6600 cameras.
FOR VIDEO
If you are going to shoot handheld VIDEO on a tele lens, then you need at the very least the stabilization provided by a Panasonic or Olympus camera, and even then you are going to need to make sure that you are very steady and strong enough that you can hand-hold the camera for the duration of the shot. 
By the way, there is a reason that people don't use the word "panasonic" and "sports" in the same sentence. (SPOILER ALERT: Has to do with their autofocus systems.)
As for video, Sony and Canon probably have the best AF performance, but both have bad rolling shutter. The 4K crop of the Canon R might actually be a benefit in terms of reach, but then it will probably be noisier than the 4K footage of the Sony a6600. 
Anyway, hoping someone else can chime in with better suggestions than mine. 

1/1000 in low light - sounds like you need an absolute low light monster for that, and a faster lens than F2.8. Going to make it tricky and expensive to find something which works better than an X-T3 or GH5.
Hard to find a telephoto faster than F2.8 for any system so you might want to look at what is going to give best results at ISO 12,800 and nails AF.
I think a Sony A7 III or A9 might be a good bet.
Canon EOS R not so much, heavy rolling shutter and not as good in low light.
Nikon Z6 would be good and in some ways better than A7 III (120fps quality, better colour) but the AF in low light isn't as good as Sony's.
Sony A6600 would have way too much rolling shutter so that's out.
I would not disqualify the X-T3 if you put a speedier lens on it but depends on how much reach you need.
Is AF important or are you doing manual focus?

I am not purposely trying to be overly negative, but that is the realm of professional sports shooters and a professional sports shooter could easily have $7,000 to $10,000 invested in a single camera body and lens (at least in terms of stills shooters). A Sony a9 II sells for $4,500 new, Canon 1DX sells for $5,300 new, and a Nikon D5 sells for $5,500 new. If a sports shooter could get away with less expensive gear, they wouldn't spend ridiculous amounts of money on bodies and lenses.
In short, shooting indoor sports in poorly lit conditions is probably one of the most cost-inducing types of photography there is. You need lenses that are both long in focal length, have fast apertures, and focus fast, and those all cost big dollars. You also want a camera that has excellent AF in low light and a high frames per second rate in stills to capture the action, which, again, are certainly far from being budget-friendly.
Purely from a STILLS perspective, you might be able to work with something like a Sony a6400 (or a6600) and POSSIBLY the Sony 85mm f/1.8 lens, but please don't quote me on that. I don't know how fast the Sony 85mm f/1.8 will actually focus, and secondly, don't know if 85mm on a crop body is enough reach for you, and whether you can live with the restrictions of a prime lens (as opposed to the flexibility that one would get from a zoom). 
As @Andrew Reidmentioned above, sony video has SIGNIFICANT issues with rolling shutter. And if you wanted the excellent AF Tracking features in video, you would (I believe) have to get the more expensive a6600 over the a6400, but again, don't want to swear by that.
Hopefully someone else will chime in with better news. I just don't want you to trade in all your gear and find that whatever you get isn't an improvement.

I appreciate the feedback.  I don't need to be a pro but I am looking for a system that is an upgrade from the x-t2 in terms of low-light auto focus in photos, low-light in general, can be done handheld without the need for a gimbal and doesn't need a rig to get images in video and photo that are above the x-t2 - it doesn't need to be a $10k system; $5k and below for the body would be suitable.