The Japanese can't really afford to garrison the Bonin Islands, at first. The USA will advance under cover of Land-Based Air - if they don't have an airbase on the Marianas zone, they probably won't be attempting to land on the Bonins. If they do early on, that is a job for Japan's reserves (the Combined Fleet, all available air assets, and their Marine counter-attack force) to congregate, cut US supply, and re-take the Bonins.
Once US LBA can fly in to the Marianas, then the Bonins must be held.
Japan should also take Wake Island and put it in their defense perimeter at the same priority as the Marshal Island hexes that border the Marianas zone. The easiest way for the USA to beat the Japanese is to simply advance straight east from Pearl Harbor, exploiting the game system and sea zone lay-out. The historical 2 prongs of Nimitz in the central Pacific and MacArthur in the SE Pacific is a waste of time in WiF.
However the Japanese can't rule out the possibility that the USA might try an advance from the Coral Sea &/or various other springboards in the CW empire to the west. These could be just diversions or raids or could become a main axis of advance by the Allies.
So a big part of what the AI has to do is divine where the main USA drive is going to be, and respond to that with priority resources. If an HQ and 2 Engineers appear on an American held Wake Island, there is little need to reinforce the Netherlands East Indies. If a couple American AMPHs Return-to-Base in Australia or India, then the AI needs to think about adjustments.