There are no third party RCS apps outside of hardware manufacturer skins on Google Messages as Google has shut them all out.

If you want to interact with the RCS world as a non-wireless carrier, expect to pay upwards of 10 cents a message and have a minimum revenue commit of thousands of dollars a month. Carriers also don't get paid for inbound texts on RCS, creating a huge new cost center instead of symmetrical texting volume resulting in minimal costs like the current SMS/MMS ecosystem.