- Like what you've got: Stick with it.
- Dislike what you've got: Stick with it. Not exactly sure what you dislike (other than the price you paid), but the only reasonable upgrade path is a used D800/D800E or D810, or a new D850. You'll be giving up body build, vertical grip, battery endurance, and more in order to gain a flash and less than a 20% linear gain in resolution.
- Upgrade to mirrorless: Z8 or Z9.