I navigate with a stem-mounted watch (currently Garmin F5, Enduro is in the mail), which provides unrivalled autonomy -- 2 long days with the F5 draw 300mAh vs 1 day on a phone drawing 4000mAh. Enduro should give at least twice (I expect 4 times) the autonomy.
Using e-ink phone as a nav device... autonomy will depend on software settings. IIRC, Locus Pro can be configured to turn the display on when required, i.e. upcoming turn, or some other course point, and off afterwards. OTOH Google maps is always on and constantly redrawing the screen unless you turn it off manually.
As I said, will update. I made a mental note to try and navigate with the phone. I may forget. Remind me
