I have used the 500 mm PF extensively on a Z7, both with the bare lens and with the 1.4x TCIII. I generally used it handheld for birds and other wildlife, usually walking around or from a kayak or canoe. I think it works quite well. I love the light weight and small size. It is my favorite lens and body combination. I did not use it on my Z6.
For everything other than birds in flight, I prefer to use the 500 mm PF (with or without a 1.4x TCIII) on my Z7 rather than on a DSLR. I find focus more accurate and repeatable, at least for me. I like having focus points that cover the frame. I also like that all focus points work with the 500 mm PF + 1.4x TCIII, even though the combination is f8. For birds in flight, I find I get better results with my D500 or D850 (not that the Z7 is bad -- it's decent even here). However, even for birds in flight, if I am going to use the 1.4x TCIII, I prefer the Z7 as I think it focuses better with the f8 combo than the DSLRs, which only use a small number of central focus points at f8.
I use the 500 mm PF on a Z7 rather than a Z6 (or now Z6II) so that I have more latitude to crop when I need to. I try to get as close to my subjects as I can, but sometimes there is a limit and cropping helps.
I have received a Z6II, but I do not yet have the Z7II I ordered. I have not tried the 500 mm PF on the Z6II. Nor have I extensively tested the Z6II, as I got it recently for Christmas. It's cold here in Minnesota.
Based on comments by Steve and others on this forum on the Z6II, I expect that the Z7II will have improved autofocus when compared to the Z7. I hope I get my Z7II in time for spring bird migration in Minnesota.
Good luck with your choice. I am sure you will love the 500 mm PF on whatever body you put it on.