How might the PRC respond to the outcomes of South Korean President Yoon’s visit to the US?

Clearly from the invective thrown Yoon's way, they aren't happy. However, if they follow that with concrete actions against Korea, similar to what happened when S. Korea deployed THAAD, they are bound to turn S. Korea even more against China.

The smart thing for China is to ignore it and try to rebuild relations quietly, particularly commercial ones, but on the diplomatic front, China seems to love shooting itself in the foot (see Lu Shaye, the Philippines, India, Japan, etc)

Though to be fair, what we see as shooting itself in the foot could be less accidental mistakes and more due to domestic pressures to show an powerful and triumphant China, especially as things get more difficult