Building a quiet PC inside an Ncase M1 is already possible with beefy air coolers, no need to complicate a build and double the costs with watercooling. At the moment I'd say the PSU's fan and GPU/PSU coil whine are the limiting factor in quiet mITX builds.
Maybe, but we all have different ways to interpret "quiet". The standard is probably set by Fractal Design R5 with carefully chosen components. SPCR tells the Ncase is still a bit behind in terms of quietness, especially under load.
Watercooling is an elegant solution, especially to silence a GPU. Otherwise you would have to find these giant after market air coolers, and special VRM heatsinks, etc. I know you did that for your build.
I still think it would be exciting to see a balanced and elegantly designed mid-size mini-itx case with silence and possibly water cooling in mind. Fractal Design is the only one getting it right on the ATX front, but it's cheap stuff they are making.