Hi everyone, new user here, but I got RCS to work. If you haven't already, try going into the settings for sandbox Google play services and give Google permission to automatically verify your phone number. As soon as I did this, RCS started working. I also have all 3 sandbox Google services installed, I'm logged in, i installed carrier services from the play store, and gave many recommended permissions to google play services and google messages. But i have a feeling the automatic number verification was the big one and may start removing permissions until something breaks.
Open settings - Apps - Sandboxed Google Play - Google Settings (logged in) - All Services (tab) - Mobile Data & Messaging - Phone Number Verification, and then toggle Automatically Verify Phone Number(s)
You may be able to untoggle it afterwards and get it to still work, not sure. But hopefully this helps people who just want RCS to work in Google messages. Part of me thinks the varied responses of people saying it works and others saying it doesn't work (or stopped working) despite similar permissions is because some people verified their number with Google and others didn't.
If anyone wants to play around to see what the minimal permissions and configurations are while still getting it to work with this, that would be super helpful.