Korea has always operated on the principle that they should tag along with a larger power, weather that's China, Japan or the US. But it's more nuanced than that.You have no grip on the reality of South Korea. There is no such thing as a brotherly love between China and SK, period. South Koreans are more anti-China and China-resistant than Australians/Americans/Indians ever are in terms of the general populace and media.
S. Korea is an extremely (pretty much the most) homogenous, closed and insulated society and they live in their own version of the world and it is incompatible with the outside world. They don't get along with any of its neighbors. S. Korea can't coexist with N. Korea, can't coexist with Japan, can't coexist with China, can't coexist with SE Asia. Period.
Since ancient time? SK is no Choson or Koryo. It's an artificial and undeserved creation of post world war 2 by the US as part of neutralization of defeated Japan.
When did Japan invaded it? Did Japan invade it continuously throughout the history? No. China had plenty of time but why assimilation never happend in the long history? The Japanese mistakenly thought that they could assimilated them by treating them as the same people and they are paying the price. Don't repeat the same mistake.
It is very strange that S. Korea always gets spared by neighboring powers and maintains mistakenly favorable image.
I think there are two reasons for that:
1. S. Korea is a small minor country in the middle of great powers. So S. Korea is always viewed less of a threat in front of bigger powers and the powers want to have S. Korea in the middle on their side to counter their bigger, more serious enemies.
Japan has supported S. Korea to counter China, N. Korea and the Soviet.
The US supports S. Korea to counter Japan, North Korea, and China.
China supports S. Korea to counter Japan and the US.
The rest of the world is nervous about China's growing power so supports smaller, less dangerous S. Korea as an alternative, while China supports S. Korea because S. Korea looks less attached to the US than Japan is to the US.
S. Korea gets favorable treatment by all parties because of their position, not because they deserve it.
2. S. Korea is such a minor, less important country so always gets little attention. Few foreigners are interested in and seriously study about S. Korea (aside from sugarcoated k-pop and k-drama fans etc). Few foreign journalists live in S. Korea to report coverage, and Korean is a minor language only spoken by Koreans (in contrast to major languages like English, Chinese, Japanese and Chinese-speaking Taiwanese and English-speaking Indians) and language barrier is very high and very few people understand the Korean language and know what's said among Koreans. That's why I say S. Korea is very insulated from outsiders. The raw reality of S. Korea less likely gets out and catches little attention outside. People outside only see the sugarcoated k-pop images that S. Koreans want to show to others.
You can already see South Korea is so far determined to not get involved between the grand chess game between China and the US. Rest assure they can see very clearly China is on the rise again and look to overtake the US in the future, so going by their guiding political principle they should switch their allegiance from US to China to get early bird discount. So why haven't they?
The reason is really simple: the people among the political class who will benefit from being a Chinese protectorate are not necessarily the same people who current benefits by being a US protectorate.
We saw this happen many hundreds of years ago when Zhu Yuanzhang's rebellion destroyed the Yuan Dynasty and replaced it with the Ming Dynasty. For quite a while Korea refused to acknowledge the new emperor and remained loyal to the non-existing Yuan. For average Korean it makes no sense right? What do they care who sits on the throne in China thousands of kilometres away as long as the new dynasty treat Korea the same as previous? Well if you're a Korean nobility who have accumulated political capital with the Mongol elites via intermarriage and so on it does matter. For them even if they managed to maintain cohesion among themselves and not have any of them break rank and welcome the Han Chinese there still will be people with political ambition watching and calculating from the side lines. Sure enough after a rousing round of political intrigue a new gang of social elite came into being in Korea and they promptly declared for Ming Dynasty.
Japan is facing this exact same issue (even though unlike Korea they've not always been protectorate of someone in their history). The ruling elite in Japan are directly descended from Imperial Japan. They know that China considers all of them to have Chinese blood on their hand and will not welcome them into a new Chinese dominated world order. They currently maintain group loyalty with symbolism like visits to Yasukuni Shrine to show each other that they're all in this together. But who's to say there wouldn't be some among them who will break rank, renounce their history in order to gain admittance into the new world order? Even if they managed to maintain group loyalty, can they really be sure a new social elite wouldn't then rise and do away with them? (violently, if necessary)