TL;DR - microG Services might be messing with RCS re-connection. Disable or delete it when trying to re-register.
So, RCS broke for me a few weeks ago, and I tried this. At first, it wasn't working, but now it is and I think I might've figured out another significant step. Used @DirtyDan suggestion to download the August 2024 version.
I accidentally broke YT ReVanced a few days ago, and when I was trying to fix it I gave up and deleted microGservices. But then I realized that might be fucking with messages too. So I here's my steps that I took after turning RCS off via the settings toggle about a day ago:
Delete Google Messages, and clear data/cache for the carrier service apps (so clear carrier services, carrierconfig2, carrier communications, and play services). You might want to do this on airplane mode but idk
Cleared cache of microG services, and disabled it (was too lazy to delete)
Downloaded the August 2024 (242499063) code via Aurora store. Don't use apkmirror or similar sites, Google will see it as a security issue or smth
Gave Messages most permissions besides location BEFORE setting it as SMS app. Then set it as default.
Since you deleted and reinstalled messages, probably will need to give it read device identifiers again. Did this with Permission ManagerX, but Shizuku or adb works too
Turned off airplane mode, so WiFi and data were enabled. Make sure VPN is off, and kill-switch is disabled
Make sure you have phone verification on under your Google account, as I did have it enabled
Enter the app, pick your account, and boom, RCS activated immediately for me, with a little pop-up and everything. Went to the settings and it says connected, and can verify that the text I sent to an individual was RCS. I don't have a group to test with unfortunately
Updated to newer version via Aurora store, is still saying connected
So basically, microG Services might be fucking your connection up. I haven't enabled it yet, and I'll have to soon to be able to use ReVanced YT and YT Music, but we'll see if it dies again anytime soon.