Yep, to make RCS business messaging useful, you'll have to use a middle-man like Twilio. They take care to sign contracts with individual RCS carriers. And some carriers might refuse interconnections entirely (as a result, Twilio supports only a subset of countries).

And of course, this is not cheap. Carriers don't have any market pressure to compete on the RCS interconnection price. So you'll be paying not-insignificant per-message fees over the IP infrastructure.