I would suggest running your own instance of Mattermost. Depending on your userbase, IRC on a channel on an existing public network (OFTC or Freenode) might work, although Freenode has been known to engage in censorship pretty heavily sometimes.

Lots of people, including Mozilla, seem to like and use Matrix, as implemented by Synapse.

The bottom of my linked post (keep scrolling) has specific recommendations for alternatives.