A lot depends on what you mean by "wildlife". If you're talking birds then you need to think about PPB - "Pixels Per Bird". When you go from cropped to full frame and you stick with the same sensor resolution you're putting fewer pixels on a bird and therefore losing potential detail. Take a look.
Here's an uncropped image straight out of a full frame camera overlaid with a grid representing sensor resolution (shaded area shows the difference between full and cropped):
You have approx. 15 boxes on the bird.
Now lets look at the same image and we'll make the cropped portion the same resolution...
Now you have approx. 25 boxes on the bird. If you need to crop in alot then you're going to want to have those pixels, even if you're only uploading to the internet.
For bigger critters it's less critical and the choice becomes more about low light performance, but for smaller things it's all about PPB. You can make up for that with longer glass but that can get expensive fast. As a wildlife photographer who shoots a pair of D500's if I were to ever go to FX I would never go down in MP's, and honestly would look for something that gave me something close to the 20.9MPs I get with the D500 in the cropped area.
The D850 will do that as would the Z7ii. The added bonus of the Z7ii is that it will give you tack sharp focus with a TC provided there's enough light. So if you've got a 500mm on the D500 you can almost get the same framing on the Z7ii with the TC1.4ii added - AND more pixels. I have the Z6ii which has 24+MPs and with the TC on there it's almost a pixel for pixel match. That said, the D500 still outperforms both as you get more fps and the focus slows significantly using the FTZ adapter with F-mount glass.