I hear you, but I don't think the reasons are entirely accurate.

They were fine with democracy both in Hong Kong and still are in Singapore. The difference? Singapore is fine because they're too far and were never considered part of China, despite the large population living there. Hong Kong, despite being even closer, is too small to be of much military or geopolitical value, and never posed much threat.

Also related - South Korea is physically very close to China (about a two hour flight over the sea), and China also shares a border with Mongolia. Mongolia is another successful democracy (well mostly, see https://thediplomat.com/2024/07/mongolias-democracy-in-action/ )

Mongolia democratized after a peaceful protest movement led to the fall of its communist-ruled one-party government in 1990. Since then, Mongolia has upheld a proud democratic tradition situated between two increasingly authoritarian neighbors, China and Russia.

Although not strictly Chinese, Koreans and Mongols are listed among the 56 official ethnic groups of the PRC, http://hrlibrary.umn.edu/research/china-ethnicgroups.html

Also, the country of Mongolia today, was part of China at some point, see https://kids.nationalgeographic.com/geography/countries/article/mongolia

The southern portion of Mongolia, known as Inner Mongolia, is part of China. The northern region became independent from China in 1921 with Russia’s help.

In other words, China seems to be fine with highly successful democracies that are right next to it, including Chinese ones and ones very closely related to them.

By contrast, Taiwan was considered the unsinkable aircraft carrier for China, see https://www.ft.com/content/22471013-a931-4e60-a8ee-29efba846ab4 and https://taiwantoday.tw/news.php?unit=4,29,31,45&post=4186

(Why aren't Mongolia and South Korea threats? Mongolia is between China and Russia, and much smaller than either, so not really a good target for someone wanting to invade. And with South Korea, my guess is that the PRC military is comfortable enough with the vast sea between them along with the land barrier that is known as North Korea, to reasonably infer that an invasion using South Korea as a beachhead is unlikely to succeed.)

I want to close by saying that I agree with much of what you point out, but it's also necessary to get an accurate understanding of what the other side here really wants. Doing so gives us a better chance at coming to a reasonable solution.