Honestly, I don’t think Japan can count on having 6 fighters after J1. You have good odds of losing a fighter in FIC to England 1 combat and another good chance of losing 1 in SZ 52. Not saying it’s 100%, but it’s a good chance in both cases.
Other then that, the easiest way to help Germany is to take out Russia.
Second is to send fighters to be anchors for German defensive positions to allow German fighters to move to the front.
Third is invade W. USA heavy and thus remove America from the war.
However, one I didnt see listed was take England’s territories. India, Persia, T-J, Madagascar, Australia and New Zealand alone is 9 IPC. If Germany took Egypt, FEQ, FWA, Kenya, Congo, It. Africa and S. Africa that’s 9 more. 18 out of 30 IPC gone for England. That’s almost enough to effectively make England non-existant.