Don, while the D750 still is a wonderful camera and the D780 is an impressive powerhouse, if you don't already have a stash of (older) F mount AF lenses, I'd strongly recommend going with a Z mount camera - you get everything the D750 and D780 have to offer plus the advantages of the mirrorless camera (though, of course, minus the optical VF).
That said, shooting with (D)SLRs has its own charm, so ... just saying. Anyhow, if it's not a price thing (the Z 5 makes it hard to believe that could be the case), I'd by now pick up a Z body and the excellent FTZ adapter and be done. While for me, there's no sensible path to getting a Z 5 (as I own the Z 6), it's the camera I'd recommend if you're not entirely sure. I think it's one of the best offers ever for a FF mirrorless, period.
If you consider video as well and do a lot of low light and rough weather shooting, the Z 6 (or, of course, the Z 6 II) is the body to go for. It's - by some margin - the most accomplished camera I've ever shot with, and it's only getting better thanks to Nikon providing firmware updates. Yes, there are other cameras from other brands that offer more in certain areas (most notably tracking AF - but it's gradual); however, overall, the Z 6 is the most well rounded offering, especially considering its current price. The only camera that (probably) beats it overall in its class is the Canon R6 - at a much greater cost and naturally without comparable access to the F mount.
M.