My current approach to this on my GrapheneOS phone is to use GpsLogger, writing to a directory that uses Syncthing-Fork to regularly send data to a computer at home. If the phone is lost I open the gpx files in Dawarich. Pretty elaborate clunky solution but works well and doesn't require Play Services.
perfect for my family who has a mix of android and iphone. getting one pane of glass to locate all of them while also preserving privacy by self hosting the historical data isn't something I knew existed until now, thanks!
I think the issue with this is that you'll never be able to have the deep integration current FMD implementations have (where you have Bluetooth beacons to track the phone even when it's "off") and also be able to use anything that requires a signed bootloader and OS (such as banking apps)
