I have the perfect solution: I bought another mount for my additional 5200 mAh Battery and this way I can keep the display on all of the time
I agree with Joachim that the Navigation feature is not really applicable for mountainbike purpose because it does not recognize turns in the right way.
However, I think that a small change to the Guiding feature could make it useful for mountainbikers who want to follow a track. There are are websites like the german gpsies.com. They let you create/modify gpx files and you can add "turning notifications" at certain points. So if the guiding feature could recognize those "turning notifications" and just wake the display up/beep on those points and not on all waypoints, this would probably satisfy most mountainbikers.
To implement such a feature, the track format will somehow have to support different type of waypoints or some additional data, to distinguish regular waypoints and turning points. I have no clue, if gpx supports that kind of information.
This way, you would have to modify the downloaded gpx tracks before following them. But I guess an automatic navigation based on the angle between waypoints will never get satisfactory results for the kind of tracks you follow by mountainbike.
This leads me to the following scenario:
If I want to follow a track with the mountainbike I choose openandromap with MTB theme in Locus.
Than I add an new route within Locus following the loaded GPX track. This is 20min additional work, but I get a clean track on the paths and I get turn instructions only when I need them.
I am also interested in what you mean by that. Probably a similar thing that I described... modifying a gpx track by adding manual turn notifications.. just in locus and not on a third party website. But its just a fictional scenario, so you cannot do that by now, right?