However I'm still interested in sms and mms encryption even it has to be only between Ubuntu Touch devices.
That has to be developed.
And my guess is it will take time until it becomes a priority, but if someone with technical skills and will wants to work on it that will be welcome.
Axolotl is a client for Signal, I don't really know if it can send encrypted messages without internet connection (but I doubt it).
Maybe if @nanu-c comes around here he might answer that and maybe add this to his todo list.
You can check Gitlab for existing feature request or make one: https://github.com/nanu-c/axolotl/issues