So the issue with RCS is that google just this last year made the move from their internal implementation of E2EE for RCS to the standard implementation based on MLS (now that the standard impl is actually viable).
They did this quietly in the background and most people didn't notice but in doing so they released new system/OS level services and started relying on those. And notably they did this rollout staggered such that only some people got it and some didn't which made everything way more confusing.
From this point on now that RCS is more or less all "standard" stuff now you shouldn't see any major breakages. The only situation where you might see one would be when android inevitably opens up RCS outside of the google services walled garden but even then you should be able to continue relying on the google services version until the non-google version is stable.
Is there anywhere public that there are writeups or roadmaps about this? I'm particularly curious about whether there will ever be an API for third-party clients.
What's left in terms of meaningful technical barriers, that would prevent this all from being baked into Android instead of requiring Play Services?