Why not get a D810? I've seen several lately with Nauticam housing for pretty reasonable prices. It's capable of getting some amazing shots and less than half the price. I saw one recently for $2,600 with cameras. A new D850 with Nauticam housing will cost close to $8,000. You would still need to buy ports but you'll have to do that anyway it sounds like. With the D810 you could get ports along with lenses and still have less invested even with a pair of strobes added in. The D850 is definitely a better camera but if you're not shooting commercially the D810 is perfect.
I've looked into Mirrorless but I'd rather wait for the next generation from Nikon where they have a chance to fix all the little issues with the first gen version. One benefit of mirrorless is the possibility for Nikonos lenses. Nauticam already makes adapters to use Nikonos lenses with Sony A7 and I'm hoping they come out with one for Nikon. when they can make that happen, especially for the 2nd gen, them I'll get it.