I’ve beaten everything possible on the 7 islands around the central one, then went back to first town (Zershear) and there was still no way to reach the central island. Is it a bug ?
You haven’t beaten everything possible then. Look further in the islands, I found that some lord was hiding in one of the islands to the right, far from the point you reach and fight initially. I had some issue like that. Hopefully it can help you.