I've discussed this extensively
within previous threads (including one of yours).
To sum up, there were two main opportunities. Specifically, in the late 14th century, Goryeo temporarily seized Liaodong (which had a significant number of Koreans at the time after the Mongols heavily promoted settlement) by taking advantage of the Yuan's collapse, and might have retained it if the Ming had not reunified the country, leading to at least a handful of competing entities within China for decades, if not centuries. The second opportunity could have occurred if Nobunaga and Hideyoshi had failed to gain influence within Japan, allowing the archipelago under a more diplomatic faction to consolidate cordial ties with Joseon. This in turn might have allowed both China and Korea to gradually expand into Manchuria over time, as no Imjin War would not have severely exhausted their resources, enabling them to successfully confront a potential Nurhaci-like figure.