Oh, Chinese wouldn't do genocide? Tell that to the thousands of minorities that dont exist any more because they have been subsumed into Han culture and their descendents identify as Han now. Genocide doesnt come just in the form of Hitler-esque quick acting kill them all actions. It can also be a slow form of moving your people in and marginalizing the natives and outlawing their language, making your own the only economic and political language, etc etc. Ask Tibetans and Uigurs and Mongolians, and Manchurians, and hundreds in the south. Look at genetic profiles of "Han" Chinese, they are more diverse than the difference between an Iranian and a Dane. Chinese have been colonizing for thousands of years.
The term genocide is historically applied to removal of native races via mass killings. The examples you raised are not mass killings, they are assimilation. Also, the Manchus have scinified themselves voluntarily. Twice. The Mongols conquered China, leading to inevitable interbreeding, partial cultural assimilation and parts of the Mongol population staying in China. Ask a Manchu if he thinks he's a Manchu, a Han, or a Manchu belonging to one of the 56 Chinese peoples. If Japan was once very much influenced by the Tang dynasty, were they being genocided by the Chinese? Japan uses Kanji, Japan has Tang architecture, is this genocide? The same goes for Manchuria and Mongolia.
Now, I'm not going full nationalist. China has committed genocide in the form of mass killings, China has committed cultural assimilation--lots of it, China has colonized. But as a nation was 4000-5000 years of history, to claim otherwise would be absurd. Nearly every nation on the planet has committed genocide, assimilation and colonization of some kind, and there is no denying it, but your examples involving Mongolia, Jiangnan and Manchuria by far do not fit genocide on the scale of western colonization.