Would I be thrown out the room for suggesting hub brakes? That would solve the brake mount issues and means for example you could get hold of an old frame designed for 27” wheels and have plenty of clearance for big tyres on 650b rims. This is something I plan to do at some point.
Before I’m laughed at all the way to the door… Back in the day in the late 70s/early 80s the first British efforts at mountain bikes had hub brakes (i forget the name of the guy but there’s a v interesting video on YouTube somewhere about it all).
Ok they weigh a bit more but they’re super reliable and won’t get clogged with mud etc., and you never have to worry about brake reach again.
At £49 what’s not to like? (Apart from one pissy review from ten years ago…)