I mentioned bitlbee elsewhere in this thread, but in case people haven't heard of it, it's a similar beast to Beeper but it bridges chat systems to an IRC client of your choice (you can use an IRC bouncer or whatever else you want to connect to bitlbee). It supports several chat systems https://wiki.bitlbee.org/ including apparently Matrix.
I mostly mention it as a historical note because bitlbee's use of IRC as the bridged protocol means that it's of limited usefulness on mobile. It was fantastic for me in the days before smartphones became a thing, but I do at least 50% of my "chat" on my phone these days.
The use of IRC as the bridge protocol is actually the best feature for me because of the multitude of client options of available including several for Emacs.
I love the text focus and at least for the protocols I care about images and videos are presented as URLs so they are easily accessible.
I'm a long time user of IRC and bitlbee (I wrote my own IRC bridge for a popular webchat back in 2004) and the last few years I've been using weechat-android as a mobile client and it's not too bad. It's still IRC, so image/video/gif attachments are out of the question, but for someone who's been using text-mode IRC clients for the last 20 years it's a godsend.
I mostly mention it as a historical note because bitlbee's use of IRC as the bridged protocol means that it's of limited usefulness on mobile. It was fantastic for me in the days before smartphones became a thing, but I do at least 50% of my "chat" on my phone these days.