I built Big Clear Activity as a companion to the Big & Clear watch face, for the same readability problem during activities.
Pick one metric — pace, heart rate, speed, distance, timer, cadence or time of day — and it auto-sizes to fill a single-field layout with the largest high-contrast digits the watch can fit. There are six high-visibility color schemes.
It is free, requests no permissions, and collects no data. If a metric or device you need is missing, tell me and I’ll add it.