Do you know about the Chinese partisan rules in Balanced Mod 3.0? They hold that if Japan leaves a Japanese-controlled non-coastal Chinese land territory unoccupied by at least one Axis ground unit, then the Chinese get to place 1 infantry in each such territory for free at the start of their turn. This allows the Japanese to garrison a territory with a stack of air units, because the stack of air units will almost always kill off the one Chinese infantry before it can roll a 1, but you wouldn’t want to, e.g., leave a strategic bomber alone on a Chinese territory, because then you’re rolling at even odds against a much cheaper unit.
I like the triplea version of “Chinese speed bumps” as mentioned above, not complicated and doesn’t require anything extra. it slows the Japanese roll through china enough.