[PetitDebardeurRose]

Throwing in my two cents:

As an admin who runs two self-hosted matrix instances: don't use matrix.  It's a total PITA, client support is all over the place with feature parity, and it's just....ew.  Cannot recommend.

Not suggesting this as an alternative, but throwing it out there since nobody has mentioned it yet: Telegram.

I will always and forever be anti-discord.  I'll tolerate slack, but prefer FOSS alternatives, so I'd be pleased with mattermost, even though it's yet another app.

Telegram has really sketch ties to the FSB though: https://www.youtube.com/watch?v=x2nYBqXGr40