also I think RCS is way to insecure for Graphene OS - they have no way of making sure the communication is actually 100% secure.
In my experience, Graphene OS doesn't make compromises.
RCS is way more secure than SMS what are you talking about? RCS+MLS (i.e. GSM standard RCS E2EE) is basically as secure as you can get unless you lock all users involved into a specific app (ex: signal, etc)
Why would they support SMS then?