Did you already try phone apps? I personally use an app called CaliMoto. It lets you plan routes and record your ride. Or you could just record you movement without planned route to follow. One other apps Rever and Ride With GPS.

I've used my bicycle gps as my motogps when I rode from LA to San Francisco, so your idea might work as well.

However, sometimes I was going fast enough to surprise the software, I luckily was able to turn off "adaptive gps" which boosted accuracy at considerable cost to battery life. Adaptive gps allows more time between gps updates by noting your average speed and calculating how long until it needs to update again to avoid signal drift. Gps updates munch battery in a heavy way, and are specifically what makes your phone hot and short on battery.

Now that I'm making a little bit more money, I'd consider a Moto specific gps. I believe beeline, having released v2 of their motogps, is selling the v1 at a good discount. Dunno if v1 is still in stock but give it a try.

It can work. I remember one of the more popular items to add a few years ago was a bicycling device that just has a single arrow pointing to the next waypoint/street turn. If you like the look, use it.