The Japanese are somewhat similar to middle eastern cultures in this respect, in that they will only accept 'help' when they are on their knees at the complete mercy of their victors.
Treating them lightly when they still have some fight left in them is taken as a sign of weakness, only after a complete beatdown, where they have no chance of further resistance, will they accept an olive branch. Do this and they will eagerly embrace and worship you as a strong big brother.
On the flip side, as soon as they smell weakness they will begin playing shenanigans. Japan's rearmament push is an example of this.
They need to be treated as a junior partner, with force, to keep them in line.
What I'm concerned about is that there may not be a realistic chance to deliver widespread devastation, which may mean once the balance of power between China and US is uncontestable (in China's favour) in East Asia, a Hong Kong NSL style approach to denazify and Sinicise Japan might be the best approach to control Japan.
Just like the Mongols, Manchus and other barbarian tribes were civilised and absorbed into Chinese society, the same needs to happen to Japan for peace in East Asia. One day, they will become one of the many ethnicities of the Chinese nation.