- Like what you've got: stick with it. D850 envy is mostly just that: envy. You bought the wrong camera if you want to trade a D750 for a D850.
- Dislike what you've got: Say what? There’s nothing wrong with a D750. Perhaps you’re trying to buy into a camera level that is above what you actually need. Either that or you have unrealistic demands for the current market. Yes, you could update to the D780, but make sure that you’re getting new abilities and performance that are useful to you, as the D750 is no slouch, and the D780 doesn’t update everything.
The Z6 II looks interesting to the D750 user, but this is future (Z6 II, mirrorless) versus establishment (D750, DSLR). Are you really ready to move to the future?
