aaravchen I appreciate that you took the time to write all of this out, and I hope it's able to help someone. But I don't believe it applies to AT&T.
At the beginning of this RCS saga, I received a text from Google saying that my carrier was now responsible for maintaining RCS, so I believe AT&T has already made the changes that Google asked for. Back in October, I also was able to get RCS working using Verizon through US Mobile, but when I attempted the exact same steps using AT&T through US Mobile, it didn't work. This again seems to indicate that AT&T has already changed something about RCS on their end.
I have since tried just about every workaround I've seen posted on this forum, on the GOS Discord, and on Reddit, and none of them have worked for AT&T. Aside from SouthernPolish99, I also have not seen anyone say they've gotten RCS working on AT&T.
I tried SouthernPolish99's steps, but they didn't work for me. Possibly because I'm using an MVNO, but as I said, I'm not willing at this point to experiment further with my main AT&T sim for fear of breaking all of my RCS group chats again.
Edit: I also want to mention, I do not believe that granting alarms & reminders permissions is necessary. It certainly is not necessary to get it working on Verizon (through US Mobile).