Japan only exists as more than a speedbump unless it takes the DEI and holds them for a few rounds.
The choice is when to try for it. The longer you wait, the more resistance you face. Japan’s game is in playing aggressive and taking risks. You’ve got ample aircraft to trade for strategic advantage, so use that in your planning - particularly in removing Chinese units from your flanks where trading a Ftr for an Inf means you don’t have to land ground units in the North to deal with them swinging in behind you.
I’ve always liked a J2 move to take Flip and Borneo.
I’ve done J1 DOW and although the momentum is strongly in your favor, it tilts quickly back to even on dicey rolls and puts you into a dogfight in China.
J3 or later DOW is going to be a simple play for Japan to march on Moscow. Problem is that you allow India to get large and they just start the shuttling of units to the crown prize of the European win: Egypt. Germany is generally in no position to hold off the UK and US while also making a march to a well reinforced Egypt by India.
The lynchpin, therefore, of the entire game is cutting India out economically. This is Japan’s sole job. Either by conquest or convoy or some combination of both. This plays into Germany’s favor because it makes Egypt much easier to claim if India is unable to reinforce Egypt.
So the longer you wait to cut down India, the worse off it gets for Japan - and later, Germany. J1 or J2 is the best DOW strategy to keep India from stopping a German swing from Moscow to Africa. And that assumes Germany is successful in Russia - which is never a guarantee for even experienced strategists.