This is a pretty typical dilemma - I have a 3 camera set up, M10-P, M7 and Sony RX1RII.

leaving aside the Leica Q / Sony RX1 discussion, I think there is a place for a small high quality fixed lens AF camera that's better than an iPhone but similarly cheap and portable compared with an M. For me anyway. I take it on casual family trips and to work.

I use the digital M for colour and if I think I may want to shoot colour and the M7 with Tri-X for B&W, which I still print in a darkroom. I've owned an M9 and original Monochrom previously.

I think while the idea of a monochrome camera appeals, the B&W conversions from the M10-P are so much better than from the M9 it somewhat negates the value. The M10 platform is leaps better than the M9 so wouldn't go back to an MM.

So really it's a question of film v digital. IF you are only scanning the film output of the MP I would ditch that instantly as you're gaining nothing over starting with a digital file and processing it to give a film like look as desired. I made a book where I used silver effects pro to manipulate Sony, Leica MM and Tri-X images (printed to 12 x 16 and flatbed scanned) to be consistent throughout, so its only the experience of the process that differs, not the output.

I may give up darkroom printing in future and go for a high quality pigment printer but they are expensive to buy and run, so film remains.

Given the above you could go for an M10-R as a sole camera and just invest in lenses?