To chat with another nostr user, we need to somehow share some information in order to connect, so that when we chat I know it is them and I'm not just picking a random person.

If so, it seems that you could associate that information with a known nostr ID in a new event kind or in a metadata field.  Maybe you don't want to make this association with ratchet keys or whatever they are called, in order to preserve privacy, but there needs to be something otherwise I'm just chatting to a random person I don't know if it is them.

So it seems there could be a way to automate this part for a better UX.

But without knowing how keychat works I can't speculate any deeper than that.