My research group still uses Slack, which is not only USAian, but also has a terrible free version and expensive subscription.
I am confident there is will to switch, but the inconvenience of learning a new platform prevents the team as a whole. What would be good alternatives that would make an easy transition period?
Maybe try Zulip? As far as I know the mastodon team uses it. You can self host it, has voip (via Jitsi) and an slack import function (according to their features page).
Zulip is great