Pixel 6a, GrapheneOS, Germany, o2-SIM.
Over a period of several months I tried all steps in the various instructions in this forum and on several other webpages. Never got further than „Setting up...“. After a GrapheneOS update RCS suddenly started working. IMO the problems are not GrapheneOS related, there are several threads in official Google support forums from stock OS users who experience the same problems. Maybe a kind of dysfunctional anti spam measure from Google.