In terms of open-source alternatives, there's Zulip [https://zulip.com]. It's very similar to Slack and fairly simple to set up. They do offer paid hosting at a subscription cost per user. However, if you install Zulip and host your own server you can run it completely for free. If you're still in the early stages of your startup you could also consider signing up for the free Zulip cloud tier to get a sense of the app.