I can't recommend Zulip enough. It made remote communication many times easier, it's a dream to use because it's faster than Slack (though that bar is so low you have to dig to find it) and Riot, and its keyboard navigation is second to none.

The mental model might take an hour or two to click, but you can think of it as Slack rooms where the only thing you can do is open new threads (can't post messages in the room itself, only in the threads).

I really really think everyone should try it, it's a big improvement.