Slack has raised our charges by $195k per year

Personally, I and my friends self host matrix for our organization but Mattermost is also a fine free-software alternative.

There are plenty more reasons to avoid using Slack, see: Reasons not to use Slack by Richard Stallman <https://stallman.org/slack.html>

Convenience is king, and unfortunately Matrix is not very convenient. Way too cumbersome to get going from a user perspective.