Using manual lenses on the Zf is easy and better than on most of the DSLRs (except for the Df?)
The main issues is handing due to the added FTZ/FTZ.2 and the concurrent movement of controls further away from the camera. Balance becomes off. For lenses up to 105(-135), not a deal breaker, but can get worse or annoying for many longer lenses, at least for hand-held shooting.
If you are convinced that 45+ MPix is required for your photography, perhaps Z7, Z8, or Z9, would be preferred to the Zf.