Every time I almost decide on my new camera, you guys post a flurry of new info (to me anyway) that takes me back to the drawing board. I'd pretty much decided on mirrorless FF. prob the Sony a7RII. But the multi page discussion here a couple weeks ago, made me pause: 1) The crop factor where a 100mm lens is like a 150mm on a crop sensor camera. 2) The lighter weight, less expensive lenses for crop sensor camera's 3. The statement that unless you're printing large images, you can't tell the difference in image quality from a FF or good smaller sensor camera. 4. Learning that Canon has the better connectivity software compared to Sony or Nikon.

My shooting will be mostly landscape, family, street scenes, classic cars. Smaller is a good thing as my wife and I are planning a 50th anniversary trip in a couple years to the UK. Plus I travel to a second home frequently. Thus the mirrorless idea.

So what would be the camera of choice considering a higher end APS-C sensor? Canon M5? Fujifilm X-T2? Another one? Would I regret not getting the Sony a7RII?

What are you doing with your photos after you take them?
If 11x14 or smaller prints or social media then IQ is virtually irrelevant as all systems handle that level of use perfectly down to the M4/3 small sensors which are very good.
For ease of use Canon generally wins hands down with less confusing menus like Sony and somewhat Nikon and others. Beyond that for Mirrorless Sony is pretty good and along with it Fuji and Olympus and Panasonic all are well established. Canon seems to be making some strong moves into mirrorless now having dominated the DSLR market for decades now. This year promises to be interesting as Canon looks to be making a move on the perceived mirrorless market leaders.
For buying right now You can't go wrong with any of those you mention and all have strengths and weaknesses. IQ is hyped but you look at how 99% of photos taken are used or not used but just filed with millions of other unseen photos taking up disk space to never be seen again you will get the needed IQ with any of these systems and most likely in the real world would never know if it was M4/3 or FF or APSC as can be seen in real world photos in publications etc.