China IS japan Soviet Union. The Kuomintang tie down millions of Japanese soldiers and equipment with barely any military aid at all. This with most of their industries destroyed and a country that still have warlords running around and a barely centralised government. The difference with China and USSR is ussr have a massive industrial complex while China has barely even complete their first five year industrial plan before japan decided to invade China.
if China has about 10-20 years of consolidation japan won’t stand a chance in hell
On the mainland, that is. Formosa and the Home Islands are effectively invulnerable behind the IJN. China would need to build a gigantic navy to smash the IJN and bring Japan to heel once and for all. But doing so is likely to alarm the Americans, as they'll realize that if Japan falls, there's no one left in the region keeping China from asserting itself as the hegemon of Asia. Worse, unlike Japan, China would have the manpower, resources, and industry to match the USA as an equal.
There's only enough room at the head of the table for a single nation to dominate the Asia-Pacific, and as far as the USA is concerned it should be them, even if it means propping up a former rival. A Chinese naval buildup will almost certainly see American-Japanese rapprochement, along with cooling Sino-American relations.