China has excellent territorial defence network engraved throughout the land. With Siberian Far East to the North, Himalayan mountains to the south, and large swaths of desert to the East, China has a formidable natural defence mechanism already in place. United States however does have an upper hand regardless of terrain advantages.
China has limited deep blue naval capabilities. Russia, UK, and France each have a formidable fleet of naval carriers which give them tremendous air superiority advantage which you clearly discounted. China's naval outreach is extremely limited to coastal defence.
China has tremendous logistics problem in navigating poorly trained PLA forces in a real conflict with Russia, France, UK, or Germany. China should be discounted as a weak but emerging great power. China is not worthy being ranked second to United States. Japan has no offensive capabilities to speak of.
The Chinese lack adequate amphibious landing crafts to deploy the amount of troops needed for a normandy style invasion you idiot. China does not possess significant naval logistical support to carry out an invasion against Taiwan let alone a nation 8,000 miles away. Think of support lines, think of methods of transport, think of the decades of modernization China needs to achieve in order to catch up with China, Germany, Russia.