It's weird to me when Google market RCS as universal protocol when it doesn't work on Android devices without Google services.

(I use GraphenOS and couldn't make it work for the life of me)

The RCS protocol is universal. Carrier RCS support is minimal, though, and third-party RCS support was never part of the spec and essentially unimplemented.

Google had to pretend to be everyone's carrier to make RCS work because the GSMA spec assumed everyone would download/install their carriers' messenger apps to use RCS, like you would back in the day with SMS/MMS. This expectation was broken the day Google allowed app developers to write third-party SMS apps, but that hasn't bothered the spec people so far.

Wait til you find out Google Voice still doesn't support RCS. (To be fair bandwidth.com runs the service under it and it feels like a product Google wanted to get rid of but was stuck with)