I want to implement that. Strava has public apis but I think they are heavily ratelimited and I am not sure they will be ready to partner with me as mine is a really small app. But I will try me best to implement it
Seems like the best flow would be to sync from Apple Health for iOS and Google Fit for Android. Then people can sync data into those platforms from any app they like (like Strava), and you can pull it in.