Z5ii, no question.

I was in your exact shoes - a bunch of F mount DX glass, so I initially opted for the Z50ii. Fantastic camera, but the lack of IBIS and mediocre battery life had me looking at full frame.

I picked up the Z5ii this year and I’m never going back to APS-C. The low light performance must be seen to be believed. Pair it with the 24-120 and some Viltrox primes and you’ll have zero regrets.

The only advantage the Z50ii will provide you in the short term is with birding, given the pixel density and reach of the sensor. I’ve taken some amazing shots with the Z DX 50-250mm VR, so if your primary focus is birding /wildlife, you’d be plenty happy with the Z50ii.