Thank you all for your replies.
They have helped me put things into context and I have given a lot of thought to my conundrum:
1) I enjoy traveling and shooting with m43rds and often I am surprised how much above it's weight it punches but there is a certain "je ne sais quoi" in the image quality of my failing D810 that I will miss. So I want to get back to Full Frame after that camera gives up the ghost.
2) There are rumors starting to swirl around a possible Z8, the upgrade of the Z7 and the Z 200-600mm. Since Nikon has been hitting it out of the park lately when it comes to lenses, I feel I might end up missing out if I jump the gun too soon.
3) I am happy with about 600mm-equivalent FF field of view if the lens is great and the pixels are solid. Also, I enjoy Nikon haptics and interface and I'd rather be out shooting than learning new camera gear. Weight is not a problem at this point.
4) Winter is slower shooting when it comes to wildlife where I live (mostly small birds in city parks).
As such, I have come to this conclusion:
I will wait until about April next year (before the bird migration starts) to see if Nikon launches the Z200-600mm and if we get more concrete info about the Z8.
If these things don't happen, I will pull the trigger on a used 500PF (they already seem to be coming down in price at MPB for example) and start saving for a Z9.
If the Z200-600mm gets released, then I will try to get it and maybe a Z6II/Z7II (or III or Z8, depending on what Nikon does).
Again, thank you all for the kind, considerate and informative replies.
P.S: Phew, GAS attack averted