Another report for AT&T in the US working with the new update and toggle!
Granted, my RCS has been working uninterrupted for the last few months, but only on a quite old version, and with the device identifiers edited to allow. Before updating to a newer version of messages would disconnect RCS and not come back.
I uninstalled my old version, restarted, installed latest messages from play store, turned the new toggle on, set messages to default SMS app, (did not turn the alarms permission on this time), started messages and it was all good and connected to RCS when I got there.
Thanks @GrapheneOS you guys rock!