So, for some focal lengths, I think you are better off with native M lenses. While I do not own it, the EFm 35 f/1.4 has been very well received. The EFm 22 f/2 pancake is a nice lens.

My issue with bigger lenses like the 70-200 f/2.8 is big lens, little grip, smaller battery to power the AF, it is an imperfect combo. Granted, the EF lens I've adapted onto my M3 the most is the EF 70-300 L. That is actually not a bad combo and you simply hold the lens, not the camera body.

Then I was thinking UWA, and I might see some uses. I think you still have the 24 f/1.4 Art. That would be a 27 mm (24 x 1.6 x 0.71) f/1 lens. I am thinking DoF would be between FF and APS-C.

In terms of light captured, a FF sensor is 1.36 "stops" larger than an APS-C sensor (864 mm2 vs 338 mm2 or 2^1.36 x 338 = ~864). So, while gaining an aperture stop you are losing 1.36x "stops" in sensor surface area. You do get into quantum efficiency here, and if the APS-C sensor is more efficient, it may make up for that difference.

Just thinking about this a little, I am coming back to the size argument for M. If you want something closer to FF DoF and light captured from a smaller body, then this is interesting.

If I only shot an EOS M, I might pick this up. Mostly for the 70-300L or maybe some nightscapes with UWA lenses. But, ultimately, this just gets an APS-C camera body closer to FF equivalent, but not FF equivalent. I shoot FF.

In terms of cost, it could be argued this is a way to get something close to the FF experience at a low price, but, Canon has also released the RP. Even though it is a bit more expensive, the RP would also be 100% native glass, 100% native AF, etc. It would take the Metabones adapter to have little to no impact on optics going through their element and little to no impact on AF going through their electronics and for Canon to have a M body with higher QE than the RP for this to be a better cost effective option than the RP, for example.