I upgraded from a RX100 II to a DSLR in 2015. I bought dual strobes for the RX100, and that helped immensely, but the focus speed (and resolution) just wasn't good enough. At least I was able to transfer the strobes to the DSLR. I had a Nauticam housing for the RX100 and loved it, so got a Nauticam housing for the D810. And then a Nauticam housing for the D850. Must get item=vacuum leak detector, whichever housing you get.
If the choice is between a D500 and a D800, I'd lean toward the D500 - mostly for the autofocus. I shot a D810 underwater for 4 years, and when I went to a D850 the difference was quite noticeable. Plus, shooting a 105vr for macro, I'm usually cropping to DX or further anyway.
Personally, with my D810/D850, I often wished for a 180mm macro lens because it seemed it was just the right distance from subject for comfortable diving approaches. The 105 forces me to get closer than I'd like. I confirmed that one day by adding a 1.4tc to the 105vr, but I didn't like the loss of sharpness or focus speed.
DX cameras have a lot more lens options than FX cameras, and smaller/lighter/cheaper ports.