... have you considered the EF-M to EF/EF-S adapter?
I own it. Have you considered the size of the 70-300L mounted on an M body? I'm not trying to be snarky, but my point is that I would like a very compact (M sized) telephoto to take for a tele option while traveling. For general purpose travel I often carry my 70-300L along with a general purpose zoom (Tamron 24-70 VC) and one of my 6D bodies, but I shoot relatively few tele shots and that makes the 70-300L a weight that I sometimes regret carrying. A compact tele on the M would satisfy that need while taking up far less space and weighing a fraction of the 70-300L's bulk. The lenses that have been developed for the M have been excellent, and a compact 50-200 tele would be a great option.
No snarkiness taken. You didn't cite the EF-M to EF/EF-S adapter in your rather comprehensive list of gear, so I didn't know you already had one. I don't disagree that a large EF telezoom mounted on the M is unwieldy. I recall a photo I saw during the early days following the release of the M, that had one with either a 600- or 800mm lens mounted ... I think the absurdity of its appearance was the entire point.
I agree with you about the weight and bulk of the bigger telezooms ... I sometimes regret humping my 70-200/2.8 IS along on trips -- frequently with the 1.4x TC attached -- when I end up not using it.
Anyway, I don't personally own or want a mirrorless camera, so I'm just a bystander here. (Nor can I talk my wife into replacing her S95 with one; it was a major achievement just to get her to switch from shooting in full auto to Program AE, so I could change the capture format from JPEG to RAW.)