I'm very curious why you dislike Mattermost, my company has been considering moving over from Slack, but we've not spent a lot of time evaluating it - they seem very similar on the surface?
You might be happy with Mattermost. It's not IRC, and that is reason enough for me to grumble about it.
Anything that help to break the addiction of putting trade secrets on internet-connected computers is an improvement over slack.
At home I recently used it to chat with open source developers about a bug. They found the cause, then my son and I fixed it.