RCS has been a royal pain for me on Android, too. Partially my fault since I'm using non-default ROMs (LineageOS on my Fairphone 4, which I then replaced with GrapheneOS on my Pixel 9a), but also mostly Google's fault for taking as janky of an approach as possible when it comes to its Messages app (which seems to be the only actively-maintained Android SMS app with RCS support, because of course it is).
The Graphene folks have at least been making progress on getting it working (my understanding is that Messages expects special permissions from Android and Play Services that GrapheneOS has to specifically whitelist without blowing massive holes in the Google Play sandbox, and without those permissions it fails to verify the phone number for certain carriers — T-Mobile included, in my case). Hopefully whatever fix they come up with works for the long haul; it was really annoying to have RCS working fine for all of two weeks only for it to immediately start failing again when the required RCS endpoint switched from Google's Jibe instance to whatever T-Mobile is allegedly maintaining themselves.
The Graphene folks have at least been making progress on getting it working (my understanding is that Messages expects special permissions from Android and Play Services that GrapheneOS has to specifically whitelist without blowing massive holes in the Google Play sandbox, and without those permissions it fails to verify the phone number for certain carriers — T-Mobile included, in my case). Hopefully whatever fix they come up with works for the long haul; it was really annoying to have RCS working fine for all of two weeks only for it to immediately start failing again when the required RCS endpoint switched from Google's Jibe instance to whatever T-Mobile is allegedly maintaining themselves.