The thing is you are talking about gouging Japan apart, they don't need to be able to ignore international law, China is. I mean going for Taiwan is a murky idea, it may or may not trigger the response the west gave Ukraine, Japan is a sovereign nation, just like Ukraine, do you think the world will turn a blind eye if China does eye on Japan?
On the other hand, Japan is not stagnated, and it's honestly unfair to compare the economic grow between China and Japan, Japan is a fully developed nation, its growth is saturated, while China still has a long way to go for development. Besides, I am not talking about economy growth, I am talking about the level of governance.
Do you know what I do in the military? I am an Intelligence Officer, specially, I specialised in Human Intelligence, I am the kind of people that talk to you and get you to betray China and tell us their secret. I literally wrote a book about HUMINT and published part of that book here on PDF. There is only a certain way to push anyone to turn against the country they are working for You are talking about Money, Cultural/Nationality differences, and resentment of the government. You need to have in road in either one of the categories to flip a person's loyalty. Again, the question I ask is, what kind of advantage China enjoy over Japan to have the resistance movement up, it's a simple question, they would have to choose China (or what you are selling to them) and not their government in order to start an independence movement. I mean who China apply the playbook to? In fact, do China even have a playbook to begin with? I mean, you can't just ask people in the street of Okinawa whether or not they support independence, and give them a bunch of money when they say yes.