What to Nikon wildlife photographers find the benefits to be in terms of more pixels, or fewer larger pixels to be? Do the benefits of use in low light, greater dynamic range and reduced noise at high iso outweigh the extra pixels and ability to crop, say with the 600mm f 6.3 lens?
Get the Z8 if you can. Having 45 megapixels for wildlife is great! It's basically two cameras in one, because it allows you to crop deep and still retain very nice IQ. If you can fill the frame, all the better!
You can see lots of wildlife examples with the Z8 and the 180-600mm at this link. I know it's not the prime, but you'll see what 600mm can get you.
You can see more examples with an 800mm f/6.3 here.
I always try to fill the frame as best I can, but there are times when I have the wrong lens on, or I screw up the framing, because I'm feeling rushed. It is so nice to be able to crop in and change the composition when needed, without losing too much in terms of image quality. Example:

45 megapixels.

Cropped down to 24 megapixels.

45 megapixels.

24 megapixel crop. Sometimes even 800mm falls short!
If you have only 24 megapixels, you'll want to be able to fill the frame every time, and even 600mm is not always going to be enough, especially with smaller birds. The extra magnification in crop mode is nice, though I just shoot in FX mode all the time and crop later as needed, if needed.
Then again if you only post to social media, you should be able to crop the Z6III files down and still get a nice photo to share.
As for dynamic range,...