I hadn't had any issues w/ a couple refurbed Nikon DSLRs (bought from reputable, authorized dealers) either: an early gen 5000 series for my kids some 8 years ago (that I used for over a year myself) and my D800 (from ~6? years ago)... and both are still functioning like new last I checked...
I didn't look into it before, but what exactly is the diff between Z5 and Z6 for $100 to matter in either direction?
Nikon being Nikon, I'm guessing there are some actual tradeoffs between them (and not a clear-cut, one-sided comparison) even though the Z6 should presumably still be better...