RCS is the future of SMS and I think it’s privacy compatible.
It gives the same features as the common messenging apps (Whatsapp, Telegram, Signal …)
It is flexible : if your recipient didn’t activate RCS service on it’s side, it doesn’t matter, the message will be sent via SMS/MMS.
Everyone is free : against a messenging system like Telegram, whatsapp etc… nobody as to download a specific application to recieve your message because everyone has a phone that can revieve SMS/MMS.
It requests a server to host the service but I prefer to trust /e/ than any other GAFAM service.
Regain your privacy! Adopt /e/ the unGoogled mobile OS and online services