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)

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.

in case you also have home assistant, you can enable location reporting too. you can request precise lock remotely and use it for buch of automations.

I found my wife phone that way

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 primarily use find my device to ring my phone even when it's on silent.

A write up of how it exactly works would be good.

SMS listener? Persistent webhook?

Read the github README

I did, and found it lacking. I had to dig around the docs to find out how it worked:

https://fmd-foss.org/docs/fmd-android/overview

It uses a persistent notification with a direct reply mode so that the persistent FMD background app can post to it.