OpenTrailPaper is open-source firmware that turns a LilyGO T5S3 board into a bike computer: ride data, offline maps, GPX routes, and FIT-file recording on a 4.7-inch e-paper screen, no phone needed mid-ride. It pairs over Bluetooth 5 with heart-rate, power, and cadence sensors, though there’s no barometric altimeter, so climbing is estimated from map-tile elevation rather than measured — a test build on a 1,500 mAh battery ran about 7.4 usable hours with the front light off. Apache-2.0 licensed; you source and assemble the hardware yourself.