Also ignoring China and letting them liberate Kiangsu or Manchuria will negate a Japanese National Objective. It takes almost no effort of Japan’s part to eliminate China. Manchuria and Kiangsu are also great for Japanese ICs. They are on the mainland so they do not require a navy to service them. They also can only be liberated by the Allies, never captured. This means should you loose them, they can not be used to produce units that will be used against you.
Also consider that Chinese infantry are “free” any other Allied unit in that area had to be bought and brought there. Which is a bit easier said than done. I see no reason not to stomp China flat from the get go.