I use a P900 and I get decent enough pics for iNat for sure a lot of the time.
In-flight is kinda hard though, yes, although certainly not impossible, at least for bigger birds. Let’s say this and this are IDable, but that’s the best I’d dare to call them, while on a good day and with a slower-flying bird it can do something like this which is not as shabby. That said, I used to use a FZ300 (much less zoom) and in-flight pics were a lot easier just by using its quick semi-manual focus button, so maybe the issue is me not using manual focus more and not the camera itself? And I’m obviously a pretty mediocre photographer who has never tried to work on technique at all, so…
If the bird is not moving, it’s mostly fine, although not super crisp at bigger zooms (example 1, example 2).
Honestly though, just being able to see and document the bird is what I’m usually after, so I happily take a bit less crisp but IDable for birds that are really damn far away, like this two smew which I could barely see as a white dot in the distance with the naked eye. Your decision will vary based on your goals :)
For zoom amount, 50x on something like a SX540 seems like it might already allow you a lot more than a non-super-zoom camera would (the FZ300 is only 24x and was already neat), so…