I run Mattermost successfully for a small-ish academic professional org, and tbh it’s been simple, straightforward, and really pretty good. I live in constant fear of the ever-nearing and inevitable rug-pull, though – they do seem to regard the open-source/self-hostable side as something they do on a minimum-effort basis only because they have to, so I would be hesitant to go down this road if I was starting again.