Tbh I have no idea. I kept up with it on the GrapheneOS discord and I think GOS put out bluesky/mastodon posts about it at one point.
What's left in terms of meaningful technical barriers, that would prevent this all from being baked into Android instead of requiring Play Services?
Well now I think the last major barrier is cleared (which was E2EE being nonstandard). Currently the on-device infra is very google-oriented but there is support for non-google-jibe RCS but it still requires google, etc to explicitly opt-in apps to get access to the APIs on device.
A big part of this IMHO is google refusing to give up their moat but they may finally get it into AOSP at some point.
If they don't at this point I think the only way it'll happen is if the EU forces their hand (which they hopefully will do).