Zulip's UI is pretty laggy in my experience (only used it on a big public server), can't seem to remember what I have already read sometimes and seems to only support threaded conversations (called topics by Zulip). I haven't hosted it, so I don't know how much work is self-hosting it. Still seems like a better idea than sending all of your company discussions to e.g. Microsoft, especially when you're not from the US and can be the target of industrial espionage by the US government.