Zulip is much better alternative due it it's threaded nature and it have nice slack import tool. Please give a try.
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I recently wrote some kubernetes charts for running Zulip for my new (smol) org, but I've ran Zulip for the last 3 years as CTO for a mid-sized AAA video game development company...
I really would recommend it over Mattermost (which was in use at another development company I was briefly a part of)