I put it another way. I've got a D850 and it's really nice actually and a new FL ED 24-70 F2.8 for it would be very nice and I could free up a bundle of funds and in this uncertain time that's not dire.
But I know if I did I'd regret it straight away. When you work with the files, you'll know what I mean. They're cleaner, more natural, have bags more adjustability in them.
I've been reprocessing older images from my D810 and D800 with DXO PL4 - as it's cracking and my feeling with a lot are - I wish I'd had the 645z then - and that says it all.
Critisms of the 645z remain small - hard to clean sensor, harder to find someone capabale of doing a really good job of getting it spotless. And that's really it.
In the mirrorless market, looking at new RF prices, Z prices, Sony E Mount G master prices, you'd be mad not to get the GF if you can make do with the lens selection and slower AF (think landscape, travel and studio) as cost wise there isn't much in it.