I have been using self hosted rocket chat on an i5 running ubuntu for my local group in DC Area at www.thinblueglue.com if you want to check it out.
I've also got self hosted streaming with nginx mistserver and other tech, chat rooms and video or file sharing. I use mostly open source stuff.
Talk to John parker on thin blue glue if you have questions.
I suggest rocket chat as an alternative to matrix, discord and slack.
Self hosted for obvious reasons. Even accessible via IP address only, can be run via TOR.
I run our rocket chat on ubuntu and it has a TOR address www.thinblueglue.com