Our massive overhaul of the AOSP Messaging app has reached the Stable channel in our App Store after 3 releases. It's well on the way to becoming our own high quality #GrapheneOS Messaging app instead. There are still parts of the interface to overhaul including widgets and important features to add. Full support for RCS with all of the additional features it brings including proper group chats, media, opt-in read receipts and much more is going to be the largest project. We'll also be adding RCS end-to-end encryption via Messaging Layer Security (MLS) compatible with Google Messages and iOS. RCS support will be provided via an optional dependency on sandboxed Google Play for activation. In the longer term, we plan to make a fully standalone implementation requiring the carrier to support it. We probably won't make an alternate implementation of using Google's own system for this. Finishing the user interface overhaul, resolving many minor issues and adding important features including search are going to be the priority before getting to RCS. In the meantime, Google Messages is already fully supported on GrapheneOS including RCS with E2EE via MLS:

Usage guide | GrapheneOS