My research group still uses Slack, which is not only USAian, but also has a terrible free version and expensive subscription.
I am confident there is will to switch, but the inconvenience of learning a new platform prevents the team as a whole. What would be good alternatives that would make an easy transition period?
It states pretty clear that free self-host is limited to 20 users and no updates (whatever that means).
Yes, but that is wrong. It is all open-source and you can run an entirely unlimited version with any xmpp server yourself.