Quick silly question , is it worth buying a Z50ii for extra reach or just switch on DX mode on the Z8 when needed , not as yet tried DX mode thus the question. Can easily turn on DX as it’s linked to one of the front buttons
The Z50II isn't as good a birding camera as the Z8 ( I have both) e.g joy stick, no viewfinder blackout, faster AF, faster shutter speeds etc).
The resolution of the cropped Z8 and the Z50II is almost identical, and there is no advantage** in "pre-cropping" the Z8, unless you want smaller files. And, at least for me, the enormous advantage of using the full frame and cropping in post is the much bigger "target area" available to find and keep a bird in the frame.
The Z50II is still a decent birding camera (much better than the Z6 I and II, or Z7 I and II) in a much smaller and lighter package than the Z8. I now use the Z50II with a 28-400 (before that the 50-250) as an "opportunistic wildlife" outfit to carry alongside my landscape or people and places outfits. Other than that, I can't see me ever choosing the Z50II over the Z8 (uncropped) in any other circumstances.
It's still a good (small and lightweight) back up for the Z8, and still useful as a second body if you need quick access to two different lens set ups, and carrying a second Z8 would be too much weight/bulk (not that I have two Z8s).
**I sometimes use the Z8 in crop mode, (I have the video button set to switch between DF and DX) but that is when I'm doing people and places type pictures needing careful compositions. This is with the 26mm f2.8, which gives me a nice pancake lens and the ability to switch between 26mm and 42mm angles of view without physically switching the 26mm f2.8 out for the 40mm f2.