I'm working on something that is best described as Komoot meets Mastodon. Self hostable and federated. It's pretty early, and the UX is pretty bare bones still.

If you're keen on checking it out you should be able to find it in my recent Github contributions, following the link in my profile.

There is a federated application called Endurain [0] that can be self-hosted, and is compatible with Strava and Garmin Connect. Not a phone app, but a web front-end. I don't know how well that works on mobile.

[0] https://docs.endurain.com/gallery/

Do you mean tracking your workout? If so there's Fitotrack on Android.