I’ve been posting some technically awesome images from the Nikon D850 monochrome lately (hopefully a few have impressed on other aspects too).
The Nikon D850 monochrome is a Nikon D850 with its color filter array (CFA) removed by maxmax.com. The NEF files are converted to monochrome DNG via LibRaw Monochrome2DNG and “Method B”, then processed using Adobe Camera Raw. Doing so avoids any demosaicing and thus retains full spatial resolution.
The lack of an EVF being the one glaring omission vs mirrorless, the Nikon D850 has the best haptics and ergonomics of any camera on the market, my opinion of course. It also lacks PDAF pixels (which I have come to despise in any/all cameras). and thus does not suffer from PDAF banding. The D850 does its phase-detect AF via conventional DSLR means via the OVF.
Therefore, if monochrome is your thing and stunning tonality with film-like grain appeals, I cannot see any better choice than modifying a Nikon D850. I would greatly prefer it even over far more expensive routes like the Hasselblad X1D, which is just too slow and with horrible usability IMO. The Fujifilm GFX 50-S/R might suit if it can be done, but the lens choices are restricted, and the Nikon F-mount is future proof.
I said “last best” in the title, because who can say if future sensors will be available without PDAF focusing pixels. Potential focusing improvements aside (Fujifilm GFX100 a notably fail), PDAF pixels in all cases do not improve image quality, either delivering image-destroying PDAF banding in too many situations Fujifilm GFX 100 or Sony A9 come to mind) or indirectly through noise suppression, leaving less than random noise patterns in dark areas.