So, this is going to sound weird and I admit very cursed but has there been any paths where Japan can ''integrate'' Azad Hind and China into each other to a extent?
By that Azad Hind is many way's one of Japan's closest allies and very important member in keeping their flank on China sealed off but it suffers from the issue of facing a much larger enemy the Republic of India who will get support from Japan's many enemies.
However there very much is a way vastly increase the amount of material Japan could send to their loyal satellite, the Burma road helped provide a lot of weapons and supplies to Chinese rebels however that become a thing that only lasted for a couple of years.
By contrast Azad hind and Japan have had decades to work on and expand the road into something far grander, Burma was officially apart of Colonial India until 1937 and had a large population of Indians until program endangered them, here with Azad hind Japan as colonial overlord has far greater incentive to favor the Indians both as a loyal group to act against any other ethnic group opposing them in Burma but maintain good relations with Azad Hind, likewise Azad Hind has every incentive for a quiet border in which Japanese support can flow through.
Likewise given the likely population explosion Azad Hind would go through, inability to expand into India given the resistance and a patron who's struggling to keep the area under control Azad Hind could be ''deputized'' to start playing a wider role in the area and settling a loyal population into not only Burma but Yunnan would very much increase Japan's grip over the frontier of China.