I have a 5d4, 7d2 and a M5. The 5d4 is fairly new, but even so I use the 7d2 more. I tend to shoot sports and wildlife. The M5 is the last one I grab to use. If I'm traveling light on the motorcycle I use it. The M5 is nice as a extra body, but when you factor in the lens adapter, it takes up about the same space as the full sized camera. If I stick with EF-M lenses then I only take the M5. Otherwise I'm packing too much stuff.
In other words, if my main bodies were Canon full sized ones, I'd go with a 7d2 over a M5 almost every time just because it meshes in better. I like our M5, don't get me wrong, but to me it is almost like a different brand since the lenses are different or you use an adapter. I just like it as a solo camera.
Hope that helps. Pure opinion though.