Third party apps like Textra or Pulse can't access the RCS APIs, if you want to use a different carrier messaging app you can't use RCS currently. Also as far as I know the system doesn't actually know what RCS messages are and they're stored as SMS/MMS internally. Like if you restore from a backup or clear data on Google Messages, all your existing RCS messages appear as SMS/MMS.
I still use Textra. I switched over to the Google Messenger app for a month or two. I ended up going back to Textra... I guess I care more about the user experience than RCS. But I would like to have RCS. I believe Google promised it years ago but never delivered.