Always regret selling the a99ii. Sold to my personal physician who as still using a circa 1998 Nikon. He was amazed
going from an ISO limit of 800 to over 50K 
Here's my favorite shot from the a99ii-
IMAGE LINK: https://flic.kr/p/VKCpWt
Buteo hears the dinner bell rs
by
MedicineMan4040
, on Flickr
So a9 AF/AF-C tracking versus cropability of the A7Riii.
For this next shot I switched from using the a9 to the A7Riii. The a9 was getting many many shots but cropping in left mush. It took many more tries
with the A7Riii but at least I got something-
IMAGE LINK: https://flic.kr/p/22n6gFM
First SEO BIF 2cd beat
by
MedicineMan4040
, on Flickr
So out west couple a weeks ago for the long distance hail-Mary's I'd use the D500 for the pixel density of the crop sensor. It gave me several keepers the a9 would not have with its pixel challenged density eh, but the a9+100-400 was primo for close to mid distance.
So David, ask yourself how far will your targets be mostly. How long is the glass you'll be using?
If I wanted a do it all it'd be the A7Riii hands down but the a9 for the mid-distance shots and for Milky Way is the better choice.
I experienced the same with the 1DXii vs. the 7Dii, many more keepers with the 7Dii because of cropability, which begs the next question-how much light
do you think you'll have when wildlifing? 1DXii would give me low light/high ISO images when the 7Dii could not.
A coworker asked me just the other night, knowing (he looks at my Flickr stream) I'm also using Nikon, if I planned on migrating to Nikon.
Here's my answer (a9+100-400GM)...from the trip out west a couple of weeks ago-
IMAGE LINK: https://flic.kr/p/2dRKR6X
Harris survey
by
MedicineMan4040
, on Flickr
Now David, after I get the a7000 ask me this question again!