>>4248023a Z5 would be nice to have for sure but that kills a lot of your budget. then you don't have much left over for lenses. a D750 would give you more left over for lenses, which are more important for photo quality.
for whether it's worth it to go full frame, absolutely. full frame is a "real" camera and if you're on Canon, Nikon, or Sony, you owe it to yourself to upgrade to it because their APS-C systems are just an afterthought / onramp to full frame. (Fuji is different; Fuji embraces APS-C, so if you went Fuji then you'd stay APS-C and have lots of options there.)
