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?

Gee i dunno, which ones have you searched?

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).

Why not straight Matrix?

It sort of depends on how your team uses Slack.

The closest thing you're going to get is Mattermost. Mattermost is not European, but it's open source and self hostable. There are multiple managed hosters out there who will sell you an instance for as little as €30 per month.

Homepage: https://mattermost.com/

Example of a manged hoster: https://www.wnm-systems.de/server/mattermost-server/ (This one's based in Germany, but I'm sure you can figure the website out)

Another alternative is Rocketchat. Same deal as before, but hosting starts at €20.

Homepage: https://www.rocket.chat/

Example of a manged hoster: https://qutic.com/de/loesungen/rocket-chat-hosting/ (also German. I swear, I'm not doing this on purpose, these where just the first ones my search turned up)

The solution you should probably pick is Matrix with Cinny. It's a federated service, so you have to pick a server. Also, adding a second device to your account is a bit more complicated, but nothing you or your group can't figure out (you have to compare emojis between the devices for the keys to sync). Lastly, there is no (voice or video) calling. It's on the roadmap and other matrix clients (such as element or commet) have it, but cinny doesn't.

Homepage of the client: https://cinny.in/

XMPP based https://prose.org/ might be an option, but it isn't officially released yet (you can selfhost it though).

I quickly checked it out and their free self-host option is awfully crippled. Pass.

You can self-host it without any restrictions. The website is a bit unclear on that.

It states pretty clear that free self-host is limited to 20 users and no updates (whatever that means).

Yes, but that is wrong. It is all open-source and you can run an entirely unlimited version with any xmpp server yourself.

Zulip is great

1. Install Prose (https://prose.org)
2. Find a provider at https://providers.xmpp.org or somewhere else

If you later dislike the provider, you can keep the software and familiar interface and just move to a new provider, you know email, right?