Okay what worked for me was the below as I had it working originally off the bat so I disabled and just re-enabled:
-turned off RCS
-cleared cache (messages and Google Play)
-deleted google messages
-downloaded messages (would only open on fresh install with compatibility mode)
-Google services framework / Google play services only have network and phone (SMS for play services)
-Google messages has network, phone, sms
-Google Sanboxed - option - messages and confirm number with google (toggled on)
-back to RCS and it connected
-Google Sandbox - toggled off confirm number with google
-Day 7 going strong and still enabled with RCS and kept compatibility mode enabled so might be the precursor here!
On Mint Mobile / Pixel 8 Pro / Aurora Store for messages app! On current GOS build!