It depends what you shoot
I do mostly small birds and 500mm is the starting point, most of the times I use 1.4x to 700 mm or even 2x in good light
Auto focus is good with z50 and Z6 even with TCs on static subjects, but not good for BIF
400/4.5 z seems an excellent lens but with 1.4x is a tad behind 500pf
If you compare prices the z 560mm is much more expensive if you consider the lower price for a used 500 pf
This is a good price for an excellent condition lens
Go for it it’s worth the money