I do not beleive it is possible to have a SLR/DSLR that is even similar to a Leica M. The whole concept is at variance. A RF is about framing and exact focus. A SLR is about seeing, thru a lens. One is for Taking, the other for Making images.
A number of cameras have been put forward. My experience and with other hard working shooters, that many of them were not reliable.
When i mean hard usage in Photojournalism,Fashion( way more rolls per session), Newspaper and Industrial.In Fashion contrary to modern ideas about film, shoot slower more carefully! Not true in my time.
Using Kodachrome one needed to bracket..
It was nothing for me to expose 20 plus rolls in a Fashion shoot, of a few hours. A friend working on a book, shot a shoe box of 120 film, on a beach, where they had flown and driven, in part of a morning and some into afternoon.I think it was about 40+ rolls.
A Leica may need attention more often, but is mostly repairable.It's less complex as well. The SLR is the more modern concept. The Leicaflex were less popular than the M, for reasons of matching cams required in different models. The R series based on Minolta, simply never fired the buyers.
One error often mentioned is the "large" size of the Nikon-F. It is not larger.
Slightly for mirror box. Without Photomic head, it is the best camera ever manufactured.The lenses were bigger than the Leitz offering, but today's Leica lenses are mostly same sized as earlier Nikkors.All my Nikkors share the 52mm filter size.28mm, 35mm, 50mm, 55Micro-mm,85mm and 105,135mm up to 200mm.The Leica filter thread once 39mm is now all over the place..The AF lenses of Nikon and others, are super-duper huge from my viewpoint.The best camera like a SLR.
The Nikon-F with ordinary pentaprism. Utterly reliable. The Nikon-F2 became as reliable a year after introduction.
Olympus while i liked the size , for me not reliable.Add to that list The Leica R4,Pentax H1~Hv were plain fragile.Olympus Pen-F /FT a total disaster..
The Pentax Spotmatic very close to the Nikon-F. It has some shortcomings.Inaccurate finder(not 100%).
The Pentax ME and MX were beautiful, tiny and largest viewfinders.
In order to make smaller, size more important than body strength.
Like perfect camera bag, it;s a lovely quest.
I own and use M's mostly. I do prefer the SLR, sorry RF fans.