- Like what you've got: stick with it. D800 and D810 envy was mostly that: envy. The D850, though, is a camera you can be truly envious of, as it improves virtually every aspect of the D6xx models.
- Dislike what you've got: stick with it. You have three choices: lose pixels and skyrocket in body quality and frame rate (D6 or Df); keep the same pixel count (D780 or Z6 II); or gain in pixels and and quite a bit of everything else for a much higher price (D850 or Z7 II). None is a sensible change, in my opinion, unless you bought wrong in the first place, in which case the D780 is probably the proper choice. Think of this way: the D600 and D610 are a less substantial D3x at one quarter the price.
Special note: a lot of folk have been abandoning their D600/D610’s because of the “dust” issue. Given that Nikon has (mostly) made good on fixing the D600 (to the point of replacing many with the D610), that’s an emotional response rather than a practical one. What’s happened now is that the D610 has become extremely reasonable in price for FX. There’s nothing really wrong with a D610, and the price/performance ratio is extremely low.
The D780 is a step up in terms of features (particularly the tilt LCD and Live View/video capabilities), but you might want to also consider a Z5, which puts you firmly in the mirrorless future.
