I went with Fuji X when I moved from DSLR to mirrorless and after I mostly did street photography for many years and now I wanted to get back into animal/wildlife. Fuji is not the solution there due to poor AF performance, so for the past few weeks I've visited my local store multiple times to play around with different cameras (they also offer rentals). I tried the Z6iii and Z8 and largely preferred the Z8 over the Z6iii for several reasons. Handling is subjective, but Z8 feels better for me (Z6iii is small and my pinky always slips off the front of the body onto the edge). I prefer the blackout free EVF of the Z8. Despite the EVF resolution difference, in reality I preferred the Z8 as well, because the Z6iii lowers resolution in live view. Here's a video I found linked over at DPreview where this is also discussed:
. The video makes it look really bad and I could not see such drastic differences with the actual cameras trying them side by side, but I've found for moving subjects the Z6iii took a hit in detail where the Z8 remained the same and looked better to me. Faster readout speed of the sensor with ES to avoid rolling shutter. Then the additional resolution of the sensor comes in handy if you want to crop heavily, otherwise the Z6iii is plenty. This comes at the price of a larger and heavier body of course... and price itself. The Z6iii is very, very attractive for the price.
Nikons bird AF is performing really well. Nothing to complain about. The rest of the AF modes, not so much. While improved compared to previous generations, I made the mistake of quickly comparing it to current Canon and Sony bodies and the Nikon is still lacking behind. So if you want fast AF performance for family, kids playing sports, etc. and you'll replace your existing glass, I'd also recommend looking at other systems.
I was pretty set on the Z8, especially because they almost have all the glass I want for relatively low prices. Two things made me question my decision. Lack of AF performance for non-birds compared to Canon and Sony (it is much, much better than Fuji!). Lack of pre-capture in RAW (only jpeg). Thought about just buying into the Z-system with the Z8 now and upgrade to a Z8ii in 2 or 3 years when it comes out. But personally I think I don't want to live with those "compromises" only to replace the body as soon as a better one is available. Looking at Canon R5 Mkii and Sony A1ii now (I find the A1ii too expensive and would only go that route if savings in glass would compensate for the difference).
That's me personally, you might want to go over to DPreview and FredMiranda where this has been discussed over probably hundreds of pages, which I tried to read all and I still can't make up my mind and have tons of questions left.
If I'd already have Z-lenses I'd just grab the Z8 now. But since I have to buy new glass, I think it's totally ok to consider other systems as well. Ultimately you're investing into the glass while bodies will come and go. It's only that switching full systems is making this such a difficult decision. Oh and, goo to your local store play with the cameras and different lenses and if possible, rent some options and take them out over a weekend.