Have you actually tried https://chat.m-labs.hk ? It only takes a few minutes to sign up, the interface and features are very similar to Slack, and many people who are the most familiar with ARTIQ already use it.
It's also ad- and tracker-free, whereas I get 19 uBlock Origin alerts about various Silly Valley spy widgets by simply opening the Slack homepage. Discord is similar. Wouldn't surprise me if they used the contents of private conversations in an unethical manner as well.
Matrix would be an option if you are against Mattermost. It's also open, free from trackers and other garbage, and one big advantage over Mattermost is it supports federation.