I am yet to read the whole 500+ post thread, but from what I've found out on my own, and gathered from reading a few relevant posts here, the issue lies in using a non-owner profile.
When I use Google Messages on the owner profile, it connects to RCS just fine, but when I do it on any other profile it has a completely blank RCS status. (Not disconnected, nor pending.) Even when I used the beta app which someone said allowed Dual Sim Registration.
Exhort14 FYI, the new beta version of messages is offering me dual sim registration now. Both are still "setting up" status for me, but maybe dual sim RCS will work for others?
So I'm unsure of how it would be done, but the solution, as I foresee it, would be for the GOS devs to somehow allow us to transfer whatever property of the owner profile that makes RCS work, to another preferred profile. Or maybe even just selecting a "primary messaging" profile where that adopts all messaging properties of the owner profile.
Another issue I've faced due to using a non-owner account with Google Messages, my own messages appear twice on my end. The recipient only gets one message, but I see my own twice. It's not a complete turn off, but it is rather annoying.