Hi David,
Having shared some thoughts already in another of your posts allow me to clarify more regarding D500 vs D850.
I wouldn't consider one as backup to the other, but more of a companion for all the reasons that MILLIREHM has so aptly presented.
For me, if reach, speed and ultimate AF is required then 500 is a no brainer vs any D7xxx/7xx/8xx type cameras (haven't used the 780 or 7500).
Unsurpassed responsiveness, buffer plus the best AF driving authority for any lens I have put on it vs my D850 and D810 make it a unique tool.
Also it offers me welcome extra DOF in macro close up.
While I have the 850 up scaled to 9fps still for the buffer and AF authority it cannot match the 500. The AF authority part is even more important to me than the buffer because it makes the shooting experience a lot more snappy and positive. By authority I mean how the lens moves from 1 af position to another without any seeking or hesitation. I haven't had the chance to compare with D4s, D5, D6 bodies which by all account have top notch AF modules.