I don't think Google wanted to gatekeep their E2EE implementation. They have some generic documentation about how it works: https://www.gstatic.com/messages/papers/messages_e2ee.pdf
The thing about RCS is that no messengers seem to care at all about implementing RCS themselves. Part of that is probably because depending on the carrier, RCS may require access to certain SIM card information, which only pre-installed apps can do, and part of it is that many developers are waiting for Google to add RCS to the same API that SMS/MMS already exposes because they don't want to implement RCS themselves.
Realistically, the target demographic for their documentation is 1) Apple (who they'd happily supply with details to get rid of the green bubble problem) and maybe 2) government officials looking into antitrust concerns. In theory someone working for LineageOS can implement an RCS client, though, but for those developers I don't think reverse engineering the remaining unknowns about the protocol (mostly "what server" and "what message contents") aren't that difficult.
I'm not cryptographer, but I haven't heard any major issues from actual cryptographers about MLS. It's encryption principles seem to be similar to those of Signal. Google is actually already using MLS in their proprietary E2EE implementation.
Ideally, MLS would be combined with MIMI so that messaging apps become interoperable, but that's probably a pipe dream.