What you said could be a solution : there is a partnership with Nextcloud. It is not clear how will be the result of this project but I hope we will be able to use the Librem 5 as a kind of server through Nextcloud (possible because we never turn off the phone, and it is always connected / less possible because it means a large disk size).
If Purism and Nextcloud can do an “out of the box” server, then we can select it for Riot.
Anyway, I still insist about SMS/MMS encryption. I think about all this people in Europe/rest of the world which use this protocol : I really hope something like Silence. Today, mobile network operators can read meta data but also content of SMS/MMS, then a few months later, you receive some ads related to what you said.
I know that a bridge to connect SMS with Matrix already exists but it is not clear how it works, if it is E2E encryption or not, etc.