It will be interesting to learn of your R5 video experience in an underwater housing.
I took some 4k120 video but haven't gone through it yet. Will be a few days until I get some time to work on it.
I was happy with the stills performance especially the continuous autofocus on bursts. I got a good keeper rate which can be difficult when you and generally the subject are moving. I am sure IBIS was working hard
It was picking the right subject most of the time automatically (on animal priority) which is interesting as there were fish, sharks, cuttlefish, moray eels plus other stuff which I am not sure were included in the deep learning algorithms.
Working out the best method to view the subject was tricky. The viewfinder covers the face sensor which means that the back LCD was never on and the viewfinder on 100%. I tended to use the back LCD for settings and optical viewfinder for shooting with my 5Div. The angle is tricky though as you (preferably) want to be slightly under your subject and my neck doesn't stretch too far! A 45 degree viewfinder attachment is an option but an expensive one. So I just switched used the LCD only. I shot wide angle so this is not as critical as it is for macro setups. I definitely loved the 45mp shots as I cropped pretty hard for some shots.
The water temperature was about 16C so quite chilly but the body has a partial vacuum so should be a reasonable insulator. That said, I had 50% battery life left and that was after 2 hours underwater. Similar to my 5Div in that respect. I think that if I had the viewfinder on 100% instead of the LCD (or both) then battery life would have suffered more. Had to get used to triggering the wake up function (half shutter press) as I was moving the camera into position so there was no delay.