Seriously, 40 bucks a month gets you a great server at Hetzner then you can have mattermost there and many other office utilities.
I’ve nothing against self hosting, but it isn’t necessarily cheaper than saas just because you can get amazing amounts of hardware for what amounts to a rounding error in accounting.
That rounding error in accounting is also a monthly charge, and it sometimes happens that you get a spontaneous demand for $50K in a week and $200K in the next year. That could buy you enough hardware to run a chat for every school hacking club in the world, and a sysadmin to manage it.