I've noticed QUIK has better support than the stock messages app. It might be worth checking out.

Could you provide the exact settings menu you are talking about in the iPhone? I switched to graphene from iPhone and ran into a bunch of mostly solvable problems. One that was out of my control was how Apple was storing my information for other iPhone users. For example, for my mom's Apple account, Apple stored the metadata that I had an Apple device and it would therefore first try routing messages over imessage and would simply fail to deliver after I switched to Graphene. I had to go into my mom's phone and manually force her phone to send the message via SMS. It correctly remembered my number for her after that, but you apparently have to do that for every person if they have the same situation. I ended up getting a new standard cell number through Cape and I use that through Graphene's stock messaging app now without any issues so far.

That is weird. I recall users having trouble after switching from iPhone to Android a couple years back - root cause was their number still being associated with iMessage, so texts from an iPhone went nowhere.

It’s possible to disable RCS and MMS. I don’t even see the option of SMS being disabled.