BMW sized the existing, rear wiring to accommodate a trailer (and is what they use), but I think anything you can do to unload the overworked brake light switch is a good thing!
The official method to get power for the trailer would be to add fuse contacts and a 15A fuse to the assigned position F2 in the main distribution box, with internal, power bolt X0 supplying the fuse. But then you have to deal with the rats nest inside the fuse box, and reaching it.
I use sheets of hook and loop fasteners (e.g. Velcro) to attach electronics without drilling holes. I try to orient the sheets so that they attempt to sheer past each other in a frontal crash, but can't.
Nice write up and pictures!