I use an Amazfit GTR 2 (before I used an Amazfit BIP)
It is not opensoure, but there is at least an opensoure software to comunicate with it:
But this is currently not running on a Librem5 or Debian11.
I actually use the watch mostly standalone. And do only synching the time with a Debian10 VM.
I started to try to change the dependency from bluepy to something available in Debian11, but have eben busy with others things.
Even I will make this usable on the Librem5, the firmware of the watch will be closed source.
Regarding degoogled Android, you could give Gadget Bridge a try. I didn’t try it in a degoogled Android yet (only Android with Google).