One does have to ask, does FF on a mirrorless body make sense? Generally people shift to mirrorless to save weight & size, both of which are lost when you have to increase everything to take a FF sensor.
....I think that FF on a mirrorless makes absolute sense. You have only to ask yourself what is the fundamental advantage of a camera with FF sensor and the answer is higher image quality. What serious photographer, whether amateur or professional, does not seek higher image quality.
Although mirrorless technology offers the ability to save size and consequently weight, opting primarily for less weight and smaller size significantly compromises physical balance when in use with very high quality lenses, durability due to materials such magnesium, a degree of weatherproofing, and importantly ergonomics < All these features are compromised adversely. The Canon EOS M5
when coupled with EF lenses is a very good example of such compromises.
The compromises of the M5 (in use with EF lenses) is exactly why I am replacing it with an EOS R (being delivered to me tomorrow, Thursday). I am very relieved that Canon's EOS R mirrorless body is as large as it is and I consider it to be the perfect size < Which I hope to confirm tomorrow when I have one physically at home.
Because I exclusively shoot wildlife (including captive, domestic, and farm animals), I need telephoto lenses which can deliver high image quality. Such lenses are relatively large and heavy and actually I prefer them that way.