If North Korea fell to South Korea, and China could get the US out of the peninsula, China would exchange an erratic belligerent poor state for a stable affluent trading partner on its border at the same time it was moving US forces away from its border. Has there been any hints that China is thinking this, perhaps that it's come up in discussions between South Korea and China or as a threat to North Korea, or ever just influential voices discussing inside China?
According to a paper from 2010 the official position of China was that it supported peaceful reunification of Korea. But from declarations to practice... And yes, China demands the departure of the USFK. That paper is written based on the declarations of Hu Jintao administration. I'm not sure if anything has changed in the Xi Jinping era in these regards, but I suspect nothing significant.
Unofficially Chinese analysts (quoted in that paper) raised a bunch of concerns, including South Korean FDI displacing Chinese one in the North, and in the long run a unified Korea being a much more serious economic competitor; essentially they fear a combo of cheap labor from North and tech from the South. Another point of anxiety is that a unified Korea might make irredentist claim over portions of the neighboring Chinese provinces that have a significant Korean minority. (China prohibits dual citizenship of the chaoxianzu for the same reason.)
To be clear, CCP is not China, but the notion of influencial voices in China outside CCP is laughable.
South Korea on its own is already a big threat to CCP because of SKorea's most valuable export, K-pop.
CCP doesn't have any incentive for an unified Korea, it would be supporting the creation of a state that is not friendly with China and a bad example working democracy right on its border for the people of China.