[nmglug] online chat systems like IRC and slack

I'd like to set something up for my students who work with me, a bit like what people have when they use IRC (which after more than 20 years I still don't grok) and slack (which is proprietary). The clickbait sites advertise 6 alternatives to slack (or 7 or 5, depending on the clickbait flavor -- https://xkcd.com/1283/ ) and I wonder if people have experience with any of them. The ones people talk about are "let's chat", "mattermost", "rocket.chat", "matrix", and I'm sure there are others. Has anyone set any of these up? I'm only interested in evaluations where people have set up their own workgroup server.