OP, i am in a similar situation as you. Long time u/w photographer (started in 2001, with an Olympus C-3030 and then switched to a housed DSLR in 2006), and trying to shoot some video as well these days.
Recently, I decided to add video functionality to my rig. I have mounted video lights on my arms, and GoPro on the cold shoe mount of the Nauticam housing. The lights are inexpensive 3000L jobbies for now, but if i find the results worth it, I’ll likely get upgrade both the lights and the video camera (likely a newer ILC that does good video as well).
Earlier this month was the first time i actually tried taking this dual rig out (well, i tried the GoPro on a separate tray with just video lights, and then combined the two systems in the monstrosity shown above).
Functionally, it works. The rig is almost neutral and fairly well balanced in the water.
But the hard part is that it is very hard to be in the correct mindset to shooting video and stills at the same time. When you are shooting stills, you are looking for the decisive moment. When you are shooting videos, you are looking for an interesting sequence of actions. Personally, i find it pretty hard to switch equally between the two (my brain prioritizes photography more).
All of us, myself included, tend to obsess a lot about the equipment part of things, but when you come down to it, photography and/or videography are creative arts and require focus. When you are trying to decide whether to take stills or videos, you are distracted. When i was using just the GoPro, i got decent video clips (atleast for a first timer). When i switched to a dual setup, I got mediocre photos AND mediocre videos.
At this point, i am glad I didn’t upgrade to a GH7 and 10,000 lumen video lights right away, as i was initially considering. I know this isnt particularly related to the specific question you were asking, but it is something worth thinking about when deciding how far down the rabbit hole you want to go,