Can't speak for Zulip, but Discord seems to be better than Slack in almost every way. It's not open-source pr self-hostable, but they are a lot more generous (e.g. unlimited message history, https://upsilon-it.medium.com/slack-vs-discord-comparison-wh...). They're also a lot faster and have less downtime. And they do have channels and threads and bots, though there may be less integrations than Slack, there are a lot.
I use Discord for some personal stuff where the groups use it, and I don't think it's anywhere close to as usable as Slack for a small business unfortunately.
