> that requires maintenance and will probably go down more frequently than Slack

Been running Zulip for two years at two companies. Never gone down, not even during maintenance (zero-downtime reloads FTW), which was twice, for version upgrades.

> don't require any maintenance (Slack)

Neither does zulip.com, which is a managed service by the makers and maintainers of Zulip and costs the same as Slack.