Vector (+Matrix) is kinda like self-hosted slack/discord with proper history sync across all your devices, file uploads, nice UI, etc.

It's really good. I recommended it to 7 people and every single one liked it, even got 5 of them to set up their own federated homeservers. We're thinking about moving a ~60-people skype group there as well.

Only issue I had is Synapse hogging the CPU and getting laggy with a large room (#matrix:matrix.org with its 4 thousand members). I'm using scaleway's 3 EUR/month Starter VC1S server for Synapse though. Hopefully it will get even better with time.