On the Linux phone I have (pinephone) with the carrier I have (At&t) there is a script that can pull and send mms messages. There's work to get this integrated into libpurple sms so it you can use it from a shiny GTK3 app just like sms.

I think it would be good to point out that MMS often doesn't even work on android phones. So making that a requirement is a bit silly.