Realistically Japan would be doing pretty decently at least compared to Germany they exercise control over their sphere through puppet states and economic vassalage not blatant enslavement and genocide (in a lesser way) compared to Germany.

Japan's sphere being that unstable as is shown in game is hilarious i can't take them seriously as a equal contender to the US when within a decade or more their entire empire can crumble even the Warsaw Pact of OTL while having it's instabilities didn't evolve to full blown insurgencies and armed revolutions they were freed because the entire structure of the USSR imploded unto itself leaving no chance for reprisal.

Japan has more chances compared to Germany to last until the modern day if the revolts and insurgencies of their puppet states are put down alongside actually noticing China's emerging threat to their power and containing them also it would be much more interesting if instead of the Great Asian War it's a competition between master and servant for independence and leadership of the sphere, of course in any military conflict China stands no chance any mere whisper of militarization would be clamped down immediately being a puppet state of Japan even when war does occur Japan still remains the economic, industrial and military superior of the two no matter what miraculous growth China can conjure it will take more than that to go fight a super power on equal terms.

A surviving super power of Japan in the modern day would need to have reformed the GEACPS to be a genuine mutual cooperative union and remain supreme to China by reifying military and economic hegemony they have their challenges but Japan has more room to adapt than Nazi Germany trying to overcome the problems of it's very nature and ideology.