It is already 4 months. IRL, 4 months is enough for a girl and a boy who spend a lot of time meeting to start dating. Especially since they had such interesting first encounter. But to us reader, those 4 months is only couple pages that cost us maximum 1 minutes, so it feels fast.
A tad bit too fast on the romantic element between kaido and carol but its cute so its alright, plus their chemistry is just drowning me