I just stepped up from an Olympus m43 rig myself. I had no prior investment in more serious gear (other than my NIkon N70 film camera and lenses that I still have around here somewhere).
I considered the D850, but went with the Sony a7rIV. To ME, all the signs point to mirrorless being the way of the future and Sony is currently the clear leader in mirrorless (and has recently surpassed Nikon in sales, to boot*).
For the OP, the options I would be considering are:
Sony a7r4 in a Nauticam housing.
Sony a7r3 in a Nauticam housing. Ideally, used, but at least get the camera itself used. Here, local to me, Ace Photo was just offering used a7r3s for <$2000. And, from them, they come with a 200 day warranty.
Sony a6400 in a SeaFrogs housing. The camera with kit lens is $998 and the housing starts at $327.
SeaFrogs Sea frogs A6xxx series SALTED LINE underwater housing
I have been using a Meikon (sells SeaFrogs) housing for my Olympus m43 camera for about 4 years now(?) and it has never leaked. The only issues I have had with it at all are when trying to use it at 135' (it's rated at 132), I found some of the buttons were being pushed by water pressure, so I couldn't really use the camera until I got shallower. And, two, the button for back button focus has gotten sticky. I.e. sometimes when I press it, it sticks in and I have to catch it with a fingertip and pull it out. Considering I have never done any maintenance on the housing at all, beyond rinsing it, I think it has been an amazing value.
For the full frame options, I specified Nauticam for the housing for one reason: If you ever want to step up to the best available option for shooting wide angle underwater, that is either the Nauticam WACP or WACP2 port. Those will only work with a Nauticam housing. I have chosen Nauticam for my new Sony for that reason. I'm not getting a WACP any time soon, but I want to have the option of using one in the future, if I decide to shell out the money, without having to buy a whole new housing.
* From Nikkei.com:
It has been 71 years since the first Nikon product name was launched in 1948. The camera business, synonymous with Nikon, is shaking. Sony is overtaken by Sony in the market share of digital camera sales, and is expected to fall into the red for the year ending March 31, 2020 (¥ 10 billion in the previous fiscal year). As the market for cameras continues to shrink, the company is looking for new growth measures, such as new entry into machine tools, while promoting structural reforms.
Making a big investment in camera gear right now, I would not invest in Nikon DSLR. I say that having just put my money where my mouth is last month. If I REALLY wanted Nikon, I would go with one of their mirrorless options and invest in the lenses that are native Z mount. Then bide my time waiting for their mirrorless bodies to catch up to Sony.... Or spend a LOT less money on an a6400 and SeaFrogs housing to tide me over until I was ready to make the big investment in full frame.