thepaperpilot

For matrix specifically, I recommend fluffy chat on mobile and cinny for web/desktop. Most notably, they both support the not-yet-official spec on custom emojis and stickers, which I think is important for any slack-like.

For the server (since you want to self host), you’d probably want to do Synapse - it supports not being federated as well as SSO. Also it wasn’t mentioned by mp3, but xmpp is another protocol that’s used by many large companies for internal chat systems as well.

Mbin?

I honestly think that philosophy is fine. Before the major social media sites all came about, the Internet was filled with much smaller communities that didn’t need to be profitable or scalable - they could be run by an individual as a hobby project. I think returning to that (possibly with the use of federation so these small communities still have a good amount of content) could keep things free, ad free, and privacy conscious