Japan has to assist with killing USSR, otherwise the Axis loses.
Build TRN’s, start offloading forces to Asia, and keep your “pipeline” full so that if Russia counter-attacks, you are solid behind your front and can keep your offensive going.
Basically, the only way to play Japan is as a relentless wave of forces flowing toward Moscow… mostly INF, backed up by your AF to provide offensive punch… and you simply “walk” to Russia 1 territory at a time using overwhelming INF and AF against a divided Russia.
Of course the kicker is that Germany has to be played in a manner that allows Germany to preserve their offensive threat on Russia. If Germany has to turtle and no longer threatens the USSR, then Japan will never be able to sustain a break in the E/N/K perimeter and will be slowly but inexorably driven back into the Pacific…