The best tipp first: BMW Navigator ablösen?! / Replace BMW Navigator?! 
I navigate exclusively with the Kurviger app (or now with Cruiser), but have fellow drivers who use the BMW Navigator. For these I create a GPX file with so many shaping points that the Garmin algorithm has little chance to change the route.
Here is my workflow: nWorkflow for Kurviger.pdf (510,7 KB) - just try it.