I use the Df but I do use it less now since I've got the Z6 recently.
What you have to decide first, is the format you want. you have already 3 DX bodies.
It's time now to go for a FX format, don't you think ? So, you can enjoy the genuine focal length of your prime lenses.
The Df is full frame/FX. In addition, size wise, is closer from you Fuji body you want to replace. AF is perfect (at least for my use) for moving subjects. I never had difficulties to focus with my manual lenses, thanks the clear viewfinder. And you can mount old Non AI/auto Nikkors which are now very cheap.
Tele converter : I was happy with the TC14 (with AFS 300mm f/4, now sold) as I never noticed much degradation. I tried the TC17, but I re-sold it quickly...
I am using now a modified TC16 with my 500mm AIP which gives AF facility to any manual focus lens.
Good luck in your choice !
Francis.