There are several caveats in your scenario that could prevent Japan and United Korea from geopolitically aligning with China.
First, given China’s "overcapacity" in the heavy industrial and ever-growing high-tech sectors, both Korea and Japan will likely remain China’s geo-economic competitors even if Seoul and Tokyo were to have foreign and defense policies independent of Washington. Yes, whilst both the United Korea and Japan would like to export to the Chinese market, Chinese companies - given its much cheaper inputs and integrated supply chain - could still drive Korean and Japanese corporate champions (Mitsubishi, Nippon Steel, Samsung, Hyundai, Daiwoo, etc.) out of the Chinese, ASEAN, European, etc. markets short of China-targeted multilateral tariffs. The Korean and Japanese markets by themselves would not be able to support the two countries’ heavy industrial and high-tech champions. Therefore, given the geo-economic competition over the same markets, Japan and United Korea would likely work with the US (likely highly protectionist following withdrawal from Asia), the EU (if it still exists), and India (China’s largest adversary by then) to prevent China from dominating the global markets ranging from AI to steel making. It would be a much more autarchic world.
Secondly, historical memories and ideology still matter. In the real world today, whilst Americans have a distaste toward China due to ideology, current Chinese distaste toward the U.S. is mainly based on US geopolitical containment of China. Putting geopolitics aside, Chinese actually have great deals of respect toward the US due to the latter’s soft power, historical assistance to China during WWII, innovation, entrepreneurship, and simply being the most successful capitalist country. That’s why so many Chinese still want to study in the US so long as they don’t get banned by Trump. If it wasn’t Trump and Biden’s brutish approach, many within the Chinese business and intellectual elites are actually quite critical of the CPC, hoping that Beijing could strike a deal with Washington to calm the waters a bit and maintain bilateral trade and personnel exchanges. The same kind of soft power cannot be said about Japan. Yes there are many anime fans in China, but this is a far cry from the admiration given to the US. Moreover, whilst every Chinese have been taught about US Army pilots fighting alongside Chinese troops during China’s most difficult time, they also cannot forget images of Japanese troops raping and disemboweling people in Nanjing. So the very of Japan and China working together against the US could still be an ideological taboo for many Chinese.
Third, Japan’s self-perception as a unique and homogenous polity long outside of China’s imperial tributary system matters. Japan was briefly an ally/tributary member of China during the Tang Dynasty right after the Tang-Goguryeo War. Yet, there is long-held belief that since the collapse of Song Dynasty, Japan became independent from the tributary system and should therefore have at least same level of nominal political equality vis-a-vis the Chinese emperor. This was shown during the Imjin War, when Japan openly challenged Ming hegemony but failed. Japan would try again in 1894 against the declining Qing and succeeded in putting an end to China’s tributary system by taking out China’s last tributary member state (Joseon Korea). And following WWII, the main argument in Japan is that whilst Tokyo lost to the US, it never lost to China. Thus, Tokyo would never accept an inferior geopolitical position vis-a-vis Beijing. To prevent itself from falling into an inferior geopolitical position, Tokyo would work very hard with Russia and India to balance against China should the US ever withdraw from Northeast Asia. That would allow Japan to import cheap Russia oil whilst selling it high-tech and heavy industrial products to India (which has long banned similar Chinese goods). As with historical memories, an inferior geopolitical position would further incentivise China to pressure Japan to own up to its recent historical debts akin to Germany's atonement of war. This would mean Japan having to pay up large sums of historical indemnities (not only to China, but also to Korea and ASEAN states) Tokyo has long tried successfully to avoid. By maintaining itself as China's geopolitical adversary with nuclear weapons and aligned with China's other rivals (mainly India), Japan could continue to avoid having to deal with historical issues way past the 2031-2045 100th anniversary mark. When you have done so many wrongs to others, you might as well continue down your path with hopes of good luck because walking back could mean the end of everything for you. It is the same reason why Russia never apologises and continues its historical imperialistic course in Ukraine today.
Finally, Japan’s geopolitical perception toward China is somewhat similar to Britain’s offshore balancing approach to Continental Europe. Just like Britain always fought to prevent the rise of a dominant continental power (Napoleonic France, Tsarist Russia, Germany), Japan also has an offshore balancing approach toward China, which is already the dominant continental AND likely maritime power in Asia. Whatever Britain feared the most is already happening to Japan. Thus, expect at least an Indian-Japanese alliance (possible with Russia) with nuclear sharing should the US ever withdraw.
As with Korea, a nuclear-armed United Korea would share a border with China and have irredentist political goals of claiming land's resided by Chinese Koreans. And there’s the geoeconomics competition for world markets. That would mean the United Korea more likely to align with Japan and India than with China.
