tl;dr: OpenTrailPaper is open-source firmware turning the LilyGO T5S3 4.7" E-Paper PRO into a sunlight-readable bike computer with offline maps, GPX routing, FIT recording, and Bluetooth sensor support, plus an optional iOS app for route planning and file transfer. It runs standalone for ~8 hours on a 1,500 mAh battery once maps are loaded, with no account or subscription required. Tradeoffs include no barometer, magnetometer, or waterproofing, basic single-band GPS, and reliance on a fiddly touch panel; contributions and better target boards are welcome.