The GitHub page for the Axolotl Mobian package now has a simple .deb package to use to install Axolotl on Mobian or other Debian-based distros (e.g. PureOS on the Librem 5). I just tested installing it and it let me configure it with a text to my phone number from jmp.chat.
With texting and SIP calling with the phone number I have from jmp.chat, and now with Signal messaging with Axolotl, I might be able to start thinking about actually using my Librem 5 and not just (mostly) playing around on it.
Since this is all about using signal to read/write messages. I read about Matrix/Riot… and there should be a way to use a Matrix2Signal Bridge. Not sure how to set it up or use it. And what are the disadvantages.