I recently sold my D750 after stupidly getting GAS for an A7rii... as reviews started coming out and performance tests etc. Ive gone right off it, for me its going to be a compromise..... I just placed an order for my 3rd D750. What a plonker! Im far worse than you.
Point is, while I was crazily considering spending £2600 on a body I also considered the rest, D810, DF, 5D3, 6D, D610 etc etc. again, but I still chose the D750, its just a wonderfully rounded no compromise camera at a very cheap price.
D810 will give you more res and better build quality, do you need that vs better noise control, smaller and lighter body, better grip, wifi, tilty screen, better AF and battery life. DF for me is a non starter for several reasons, not enough MP, poor AF in comparison, clunky controls and handling, no video, only 1 lens actually matches the old school looks.
Thank F I kept my Tamron 70-200 VC! Now I need to order some more primes.