I tried it with build 2025100901 and the latest Google Messages from Play Store but no dice. Started over with the older Messages described above and RCS was already connected by the time I got to its settings. Messages app updated through Play, a reboot for extra measure, got the RCS prompt and RCS messages seem to be sending and receiving. Pixel 7 Pro on Mint Mobile.
Twlight-Princess I did try the older version, couldn't even set as default sms got a secure privacy error ?
That's a restrictive permission that can be un-done in Settings/Apps/Messages/Permissions then upper-right 3-dot menu. Once it says 'All permissions' Messages won't be greyed out for making it the default SMS app. Using adb to install the apk instead of tapping it will probably get around this.