That said, I suspect that Strava and Apple (and likely even Google) have more data than Garmin. Maybe quality of data is lower (garmin is higher % athletes?).

There are a lot of companies planning or starting to tell you about fitness. Oura and Whoop and now Fitbit are happy to give you basic training readiness info. Google will help you plan runs, Apple will give you “training load”.

I’m not sure what your target market is, but don’t forget to look broadly. Garmin is favored for training athletes but fitbits and Apple Watches are favored for casual workouts. If you’re trying to train models on metabolic or other physiological training ability, don’t forget to look at non-athletes or early-athletes.