There’s AsteroidOS but I couldn’t find any of the supported watches (all quite old IIRC) at a reasonable price.

Gadgetbridge with some proprietary watch is fine privacy-wise (I had an Amazfit GTR3 pro, I needed to register an account with the Zapp app and use it once, but then uninstalled it once I got the required password and used Gadgetbridge exclusively).

Bangle and the Pine Watch are low-res and IMHO quite ugly compared to alternatives from big brands.