Are there any mechanically coupled solutions that can be field installed by the owner?
There's a bunch of motors installable instead of the bottom bracket. E.g. from Bafang. You just connect it to a battery and that's it. The controller's inbuilt.
Swytch Bike is one. Hub motor.
There's a bunch of motors installable instead of the bottom bracket. E.g. from Bafang. You just connect it to a battery and that's it. The controller's inbuilt.