I'll just jump in to say that in general this forum has a lot of sony lovers - and for very good reason. I ended up going canon for a hybrid shooting solution with heavy video focus and I would definitely do it again.
I will echo what has been said, that especially with full frame, things start getting bigger and more expensive constantly. Dome size and f/stop all become really important and some of the WACP/WWL solutions help that in a few different ways, and the smaller port size of Sony's (on Nauticam) help all of that by about a stop. Personally, I want to shoot everything at 60p and Sony's implementation of that on the A7iv as a crop, removed that from my interests. A1 - too expensive but very nice. And A7s III is an interesting option but didn't make the cut. Any camera that crops to do 60p means I am spending a whole lot of money on lenses and domes and full frame, not to use a significant point of it - which removed many of the Sony options.
What is not always talked about is low light performance of full frame sensors and in low light the ability to shoot at F/14 - 1/125th and with a crazy high ISO and get clean 4k is really remarkable. You will love that coming from the gopro!
I shoot an R6 (not ii) above water - and love that camera for stills and just about everything video except I hate that one doesn't get full control of the settings for the red record button when in stills mode. You have to rotate the mode dial. In addition, I love love love that the R5 (what I shoot underwater) does not require a significant dial turn to change modes - its just two button presses. Underwater I like this since the mode selection dial has been the part of the housing that has had to be replaced on multiple of my camera housings. I think the R6ii has a switch - which is great.
I also shoot with both a 180 and 230 dome. And in video? I find the 180 is pretty solid for a lot of things. I can dig up some video I shot this way if you are interested. And if you shoot at 20mm - it makes things a lot easier from the dome size perspective. But if you don't need the corners to be epic sharp (and often they are out of the depth of field plane) then you can shoot much wider when the scene calls for it.
While I don't know anyone who has used it, the R7 also has a lens (well 2) that can work with the WACP-C. The R6 does as well but it essentially is a 28mm fixed lens since none of the zoom range works. Plus you can put an RF-S on any RF mount and it just shoots the crop center - something I didn't appreciate when i bought into the canon RF system (I am a long time canon user). No one needs to point out the contradiction in me complaining about sony instilling a crop to shoot 60p, and then recommending a crop body and also shooting in crop mode by using an RF-S lens...the internal conflict is real. (you are also welcome to point out the contradiction)
The other camera I would seriously look at, if I was you, is the FX3 and FX30. If you are really shooting video pretty much entirely - these seem like really interesting options. In many ways the A7sIII has a lot of similarities to the FX3, so that is something to consider. I think the bodies are identical so you could get the FX30 now + all the housing stuff and then pick up a FX3 later? Not sure on this but something I have been thinking about.
I'm also pretty interested in the body announcements coming down the pipe. If the Canon R8 is an A7c (ii....hopefully soon) then that may be a real gem for underwater use.
Finally, I will say the jump to any of the cameras discussed is huge. I still bring a GoPro 11 on many dives and rarely touch the footage after it. The Canon (in my case R5 underwater) is just a different level of quality. I feel the same about the R6 above water - it is just wonderful and by all measures, for wildlife video, the R6ii should be even better than that.