There is Mattermost which you can self host.

Once you do, you will realize the DB grows REALLY fast

I poked around with Mattermost like ~8 years ago, but never anything serious. I don't know how good it is now, especially w/r/t administration, but I have to imagine that if you're concerned about $1000s -- let alone $100ks -- in annual costs, you can scale up your storage and still come out _way_ ahead. Maybe that's a naïve take?

Until you have to administer the DB. It’s doable, of course, but RDBMS administration for anything beyond toy scale gets interesting.

Our enterprise deletes all PMs after 24 hours. That's one way to not worry about that.

I think this is a good approach, although 24 hours seem very short. 24 hours is not even covering overlap across time zones, WE or PH.