Dear David,
Too much choice is the bane of all of us
Owning currently D850-D810-D500, I would first let go the 850 and just maybe the 500 (if a gun was put to my head).
So I would propose you to consider (if you have not already made your mind up) the D810 as the back up, unless you need the reach, speed and buffer for wildlife and/or sports in which case the 500 cannot be beat.
As to how to best utilize your ai ai-s arsenal I strongly propose going mirrorless, but on a different route. Olympus EM1-II with either simple or speedbooster type adapter. Imagine the best IBIS making your lenses instantly stabilized with excellent EVF assists and for close up macro one of the highest pixel densities available, especially if coupled with the amazing high res mode (works also with any Nikon I believe).
That is how I use my 55 3.5 and 2.8 macros as well as the 50 1.2 ai-s and all my Zeiss glass.
Let us know what you ended up with.