You should be able to opt out at the brand level and I believe Googles unsubscribe button in chat should work...
Good rule of thumb for RCS don't give your phone number out to companies or websites unless you use it for a service that require it like 2fa, remind, loyalty cards, Receipts, delivery statuses... And unlike email if you unsubcribe and block that RBM agent they should leave you alone because it's not like email where they can just spin up another one quickly from what I understand.
That's more of a rule for RCS not a rule for privacy