Personally I've found Matrix significantly more user friendly than Threema work. Zulip I haven't used in anger so I can't comment on that but I've seen a few places that even open source they charge per user for things like notifications. Not ideal IMHO. There should be an option to replace notifications with a separate service.

It's hard to find a decent service that ticks all the boxes but I do sincerely hope that the EU can support Matrix to bring it up to the standard that we all deserve.