> Is there anything similar to (or better than) overleaf for collaborating on typst docs? To be clear, overleaf isn’t all that great but I sometimes work with groups that are used to it.

Yes, https://typst.app/ is the official Typst app (part of how they make money).

> Have conferences that traditionally accepted latex source (and specified latex templates) started accepting typst as well?

Very few by now.