GrapheneOS I defintely hear what you're saying, but I'm on Messages 20250813_00_RC02, installed from Google Play. I assume that means that update came on August 13. I have had many reboots since them without a problem. There were two recent updates of Graphene, since the beginning of September. After the first once, RCS still worked for me. But after the Graphene update I got a couple days ago, RCS stopped working. It was working this Monday, but stopped working yesterday, September 9. So my experience doesn't seem to fit what you're describing.
I'm not blaming Graphene. Obviously ultimately the problem is, as you say, that RCS is not an open standard and it requires Google Messages, along with all of Google's onerous verification procedures. But that does not mean that an OS update can't break things for some people, for whatever complex reasons.
I do think that given how important RCS is to a lot of people, it would be nice if Graphene tried to work on ways to make it functional, in the same way that Graphene tries to keep Google Play working and provides a compatibility layer for that, even though Google Play is a third party app and service. I wouldn't suggest this about anything else, but for better or worse, RCS is the future of stanadard carrier supported messaging, and it really out to work on any OS, even if that means dealing with Google's crap. I don't know if technically it is possible for Graphene developers to do this. And maybe they don't want to. But it would be nice to put some focus on it, if possible.
At some point, no matter how much this is Google's fault, if RCS works on every phone, including iPhones, and is a de facto (albeit proprietary) standard, of course we're not there yet, but phones with GrapheneOS are the only ones where RCS doesn't work, that seems like a problem for GrapheneOS as a viable alternative. It could get to be a situation where having RCS not work would be tantamount to having SMS not work (especially if someday they phase out SMS). That would obviously be a non-starter for almost everyone.