https://f-droid.org/packages/org.fossify.messages/

https://github.com/FossifyOrg/Messages

There are no other RCS clients, just Google Messages.

I don't have an answer for you, but I have a caution...

I once worked for AT&T and worked on AT&T Messages.

DO NOT USE IT, if it still exists, if you're with AT&T. At the time I worked on it, there was no encryption except in-flight (https) -- which means if I had had production access (and some people who worked there at my level, definitely did), I could have read all messages, blobs, everything. I was told after I quit that they intended to add encryption, but since AT&T would still hold the keys, it's useless.

Silence - a Signal fork is actually still quiet decent. Simple and support for encrypted SMS. Which is a fun feature.

If you're in America and have Verizon as a carrier, Verizon Messages is one RCS alternative to Google Messages. RCS apps are marketed as "joyn" or "Message+" in other places as well, but most of them have been shutting down after about a decade of service because nobody but Google cared about RCS.

Until someone makes an RCS app for Android, you'll have to use Google Messages for RCS functionality. Most messenger apps seem to be waiting for Google to add RCS to the SMS/MMS API rather than implement RCS themselves, though, so it'll probably be a long wait.