And that irony now is that messenger on Android is RCS compliant and currently has this exact functionality, except it’s less trustworthy.
Once again I’m using one messenger and everyone else who’s using an RCS messenger gets encrypted, but SMS (clearly marked as such) is a viable fallback.
GYB is very powerful and flexible, and I’ve used it to manipulate Gmail accounts in the hundreds of gigabytes.
The community is decently supportive, but of course it is a command line tool so some familiarity with the shell is useful.
https://github.com/GAM-team/got-your-back