If you do go for Bangle.js, ping me if you get to the stage of putting any code up on github - I'd be very interested.
My second impression about the Bangle.js is: "oh no, JavaScript only?". Still, it's much better than building the same thing from scratch, except it may still take effort on the software side.
Having another interested person involved would be amazing for my motivation! Btw, one of the reasons I wanted hackable hardware was to map road surfaces on OSM ;) GPS alone doesn't add much to OSM in my area any more.