Possibly split Messages and SMS in to distinct apps.
Have the latter handle SMS/MMS/RCS with options (in its settings area) to enable/disable each of the three, such that any or all of them may be individually allowed (or not).
Then also have an additional pair of options in the SMS app to indicate which other message app can operate as a proxy. Said proxy being able to send or receive SMS/MMS/RCS as appropriate. Possibly have the default set to Messages, but allow it to be set to any other app which has opted in.
Remove all of the SMS/MMS options from Messages, except the 'Send as SMS', which would then try to do the existing fall back when there is no data service.
The default behaviour would be as now. Disabling the 'Send as SMS' option would keep Messages and SMS as two distinct services, and one could then run the that way. Further flipping the config in SMS would allow any other app to be the preferred primary contact point.
I'm not sure how one would handle 'Group Chats' in such a situation.
With such a situation, I'd split the two and operate as distinct systems, iMessages in its own app, SMS/MMS/RCS in the SMS app, all other message facilities confined to their own playpen.
I know some relatives who would prefer to keep the proposed defaults such that everything appears in the Messages app.
Have the latter handle SMS/MMS/RCS with options (in its settings area) to enable/disable each of the three, such that any or all of them may be individually allowed (or not).
Then also have an additional pair of options in the SMS app to indicate which other message app can operate as a proxy. Said proxy being able to send or receive SMS/MMS/RCS as appropriate. Possibly have the default set to Messages, but allow it to be set to any other app which has opted in.
Remove all of the SMS/MMS options from Messages, except the 'Send as SMS', which would then try to do the existing fall back when there is no data service.
The default behaviour would be as now. Disabling the 'Send as SMS' option would keep Messages and SMS as two distinct services, and one could then run the that way. Further flipping the config in SMS would allow any other app to be the preferred primary contact point.
I'm not sure how one would handle 'Group Chats' in such a situation.
With such a situation, I'd split the two and operate as distinct systems, iMessages in its own app, SMS/MMS/RCS in the SMS app, all other message facilities confined to their own playpen.
I know some relatives who would prefer to keep the proposed defaults such that everything appears in the Messages app.