Use Zulip. The migration out of Slack is actually quite easy and preserves all messages, files, etc. Even the user migration is straightforward, keeping Google or whoever as the identity provider if you prefer.
The Zulip team has been admirably cautious with their own approach to AI in the product - which I am so thankful for! - but I am sure someone out there has built the integration to get bots deeply into a Zulip org. And if not, building that integration is so much more achievable than rebuilding the whole of Slack.