@Draq & @TimG,
Being lucky (daft?) enough to have both a D500 and D850 (both housed in Nauticam housings) there are really only 2 scenarios where I lean towards shooting with the D850 over the D500.
1. Wide angle where I can shoot the WACP on the D850 (with the old 28-70mm lens, for example). I don't take the WACP on every trip, and when I do it means lugging around another piece of luggage just to hold the WACP (small to medium suitcase just for it). The optics are really nice however (w/good corner sharpness), so if you there's a good chance to find large(r) subjects it often makes sense to bring it (like in the Maldives this summer).
2. Blackwater diving - the D500 with a 60mm is very good, however I've found that the D850 with (the same) 60mm is virtually as good focusing, and you can crop into the image a little bit more, so you tend to keep a few more shots.
For regular macro I actually prefer the D500 - I find it to be more flexible framing subjects and it's really good in low light or where a subject is being a bit elusive. Also, shooting anything "fish-size" I prefer the D500, might just be that I've shot with it longer, but it feels a little bit more flexible.