Slack has raised our charges by $195k per year

We're using teams in my new company, which is awful for textual communication (lacks threads in chats, groups are more like old forums than new IM). I've been experimenting with self-hosted Mattermost but it seems that it also requires paid license in some situations (e.g. does not have groups for some reason in the free version).

I was unable to find another system. Would anyone recommend me something?

I get the sense that Mattermost is the same kind of eventually-get-you-paying play as Slack.

Other threads are mentioning Zulip, which feels more old-school free as well as Free open source.