Hi Mark,
Sorry about your drama. Never nice to have a flood!
From a fellow Nikon shooter, that has been down the path you are looking at, I can add the following: I would certainly not look at getting anothe D500 and you will just have problems finding part, housings etc etc. Mirrorless is the future for now, and I would definitely suggest getting the Z8 over the Z6ii. I have both, and the Z8 is way better. I am even looking at upgrading my housing to the Z8. You will not regret getting the Z8.
With regards to housings, there are a few good options. Unfortunately I highly doubt S&S will be bringing out a housing for the Z8 - I would not wait. So you looking at either Nauticam, Aquatica, Marelux, Ikelite, Seacam or Isotta. Then it comes down to personal preferences: plastic vs aluminium, design, and of course, budget.
I have used most Brands at some stage, and they are all good. I had the Nauticam D850 housing and it was awesome. Unfortunately it got stolen and I couldn't afford to buy another Nauticam, so I now have an Aquatica, and it does the job just as good. Before my Nauticam, i had Sea & Sea, together with all the ports. I then got an adaptor for my Nauticam and used my S&S ports on the Nauticam and it was 100%. I even use my S&S ports on my Aquatica. It is obviously better to get the ports for each Brand from that Brand, but then again, it comes down to budgets etc. My S&S ports worked fine with Nauticam, and they work fine with Aquatica. When I have the funds to upgrade, I will buy Aquatica ports, but for now the dome from S&S does the job, using the port adaptor supplied by Aquatica.
Hope this answers some of your questions. Shout if you need anthing else.
Regards
Anthony