Yay, another great update!
I don't think he westernized yet, he just had the winds of westernization? He still needs the prestige and military score.
There is no possibility to demand anything drastic from the Chinese, you'd need more warscore than 25% and to do that you'd have to occupy parts of their land and they simply have too many brigades for fighting a successful offensive war. Defensive, with the trick in the mountainpasses, is doable. But don't pass by any opportunity to keep them humiliated yeah, it keeps them from westernizing which is good for you in the long run.
On the other hand, if you can take something off them now before you actually westernize, that province also turns into a state (not a colony) when you do. What are other good provinces that might be releaseable from China? Taiwan isn't too much score right? Plus its not next to hundreds of brigades and can be defended pretty effective. You probably don't want to release Xinyiang because it hampers your ability to influence them later (+50% for being neighbours).
If you are better equipped and with more men later, then you'd better release Yunnan as that prevents the British from influencing them too much (Assuming they took Burma) or alternatively prevent the Russians by releasing Mongolia and Manchuria and then taking that proto-Kazakh (Kokand?) state for yourself. Russia will eventually take the Chinese part of Sofianova and Primorsk (hopefully) so they will lack borders. The only thing you'd have to prevent then as well is the British from coming up via the Himalaya and taking Tibet for yourself. Anyway, /some random thoughts.