Decent email is significantly better than slack at the actual asynchronous part, but if you wanted something chat-like zulip is decent though I'd argue the whole chat-like sphere is never any good at asynchronous.

The issue isn't people being in a different timezone in a one-to-one setup (that's easy to do), and if direct answers from a group are needed then it's not too different, but having any kind of sensible asynchronous discussion on slack is basically impossible (having suffered through far too many). Email (and to a lesser extent forums) are far better for this.