I'm very interested to hear what your impressions of the Z50 are after you got to use it a bit more. These shots at high ISO look very good indeed, I'd say they look even better than shots taken with the D7500 at high ISO.
Even though the Z50 was criticized quite a lot in the photography forums for all the things it doesn't have, like IBIS, I think it's a very interesting camera that has a lot to offer: Excellent sensor, small and light wuith good ergonomics for such a small body, good weatherproofing, focus peaking and so on. Combined with AF-P 70-300 it looks like a very attractive walk-around camera. Plus it can be used with old manual Nikon lenses, like the good old 5.6/400 IF-ED.
The big question is whether the AF is good enough for birding. Is it on the same level as the Z6 (which I know a bit), or is it better? And how bad is the EVF lag? How long does it take to wake up from sleep?
Hermann