What would be cool if there was a very simple low power display bluetooth which just displayed an left/right/upp arrow with a number counting down when to turn or stop. Phones don't really work mounted on a bike, but a simple display could do the job. Sort of how android watch displays an arrow, but using a low power screen.
I feel you - I have tried attaching my android watch to the handlebars and functionality wise it's perfect UI for navigating. However it takes about 10 seconds for the screen to dim to the monochrome power saver mode and then it's impossible to see anything during daylight. Android wear does not support a permanently on screen all the time and I would totally buy a cheap device like that just for navigating.
Buy a Pebble? I have been using a Pebble 2 SE for over a year and one of the primary use cases is so I can navigate on motorcycle.
I leave phone in my pocket. I set the route, maps sends a notification before turns and the watch picks it up. Works in full sunlight and I shake it at night to light it up.
Gets 5-6 days of battery life, I think I paid $40 for mine.