Also consider Nextcloud, which has a "phone track" plugin and there is a corresponding Android application that works quite well on GrapheneOS. Nextcloud Phonetrack is also compatible with several other Android applications that can provide it with location updates.
I'm actually just getting into this one myself because my kids are starting to venture out on their own. Handed them a Pixel 3a with GrapheneOS (Android 12, unfortunately no more updates for it), and a data-only sim card, with phonetrack sending their location every 30 seconds and nextcloud talk. Battery life is surprisingly excellent -- even with the GPS continually on, it'll run for a couple of days. Well, as long as they don't do any video calling, then the battery drops like a rock.