to start General War with Japan I build just one TRS and one AMPH, on schedule for Sep/Oct 41, with maybe one more of each 'on the way' partially finished, ultimately depending on what turn war starts.
For Japan, I empty their lnfantry class pools in 1940 to make maximum progress in China and keep an option open to get some easy goodies from the Russians, too. Japan, like all major powers in the game except Russia really, can never have too many troops. Russia can build too many troops and then hit the "Baltic States Funnel" and be left with a permanent second line of lower factor units that never see land combat again.
Once war starts, the Japanese aircraft designers seem to go into overdrive in presenting new models to the Emperor, and many of them are not improvements on the triumph of the Mitsubishi A6M. But if you want to keep those lovely A6Ms on patrol far out over the increasingly Yankee infested oceans, you have to indulge all the heads of the Zaibatsu and politely purchase their aircraft whether you want them or not, so they can save face when you come back later and say, uhh, guys, we need you to switch your factories to all A6M, all the time.
And then you are left with a bunch of really so-so air combat fighters with pitifully small fuel tanks for war in an ocean as big as the Pacific. fortunately, the Wizard of Australia who is really in charge of your fate has a solution to help you - it seems patriotic partisans in Korea and Manchuria are greatly awed by the possibility of being strafed by these FTR2 that can't really stand up to the horde of Hellcats on the horizon. So you can deploy Japan's slightly expensive garrison option - Pilots, and free up more troops for the really really long front line in China, where they are really needed. And then you can keep those A6Ms in heavy production, which you will need, because those Hellcats are fierce. Be especially careful if your pilots hear the call sign "Baa, Baa, Black Sheep" on intercepted Allied frequencies. Your pilots hearing that don't stand a chance, that I can tell you.