https://www.opensourcealternative.to/ lists rocket.chat as an alternative to Slack and Discord
I have not used it. I have used Mattermost. And it can do channels, teams, hashtags, @ mentions etc similar to Slack. Though it requires the server be hosted and administered by you or someone you know.
+1
And
In the off chance the files are not under git or some other VCS, might be a good idea to add the -b option to backup