I do very little post-processing since I started using the Df, in 2014.
This holds true no matter which lens I use - old MF Nikkors, recent Zeiss, or Voigtländer.
I did some shots with the Z7, combined with a Summicron-R 50/2 (II), and was happy with the results.
Bottom line, it may well be that self-complacency is the dominating factor here. But there may be secondary factors. For instance, as brilliant the photos were with the Olympus E-M1, they felt ever so slightly "overprocessed" and that made me stick with the Df. The optics were not to blame, I guess.
Concerning the optics, it seems that I can cope with most of them, e.g. the 50/2 - Nikkor AI, Summicron-R (II), or Zeiss Milvus: these belong to different generations and cannot be more different in any respect, but each one has its set of "use cases". Different souls, but none is "soul-less".
I guess I spend as much time carrying the camera as I spend watching the results, so maybe the biggest difference lies in the "user experience". With the Df, I'm interacting with a camera; with the Olympus, I was interacting with an admittedly fantastic mini-computer. Same with lenses. Maybe that could also explain your feelings.