> not something that can reassure China's neighbors that this is where its desire of expansion will stop

May I ask if you actually live in one of these neighbouring countries? I do -- in fact I have lived in more than one -- and I can assure you that many/most people living in these areas outside of the Western media bubble absolutely do not share your view.

From the CCP's (and many Chinese people's) perspective:

1) the U.S. repeatedly interfered in the CCP's/KMT's attempts to resolve the civil war -- see e.g. the First and Second Taiwan Strait Crises (during which the PRC shelled Taiwan), Project National Glory (the ROC's plan to reconquer the mainland) -- preventing the mainland and Taiwan from reunification;

2) the Taiwanese government has lost the civil war, and the loser doesn't get to set the terms.

Pretending that the PRC's interest in Taiwan isn't special is to ignore extremely crucial historical circumstances that are core to understanding the situation today. Regardless of what you think of the PRC's stance on reunification, their desire to reunify doesn't exist in a vacuum, and it takes ahistorical leaps of reasoning to suggest that the PRC might want to annex South Korea, Japan, Vietnam, the Philippines, etc. next.

> only during the last few centuries

This is way more than enough time to drastically transform the culture of a society. Taiwan today is culturally much more similar to the PRC than it is to the West. In some aspects it is also similar to Japan, despite the fact that Japan colonised it for "only" 50 years.

>Taiwan today is culturally much more similar to the PRC than it is to the West

The cultural distance between Taiwan and Japan, Korea and Hong Kong is less than the distance from mainland China. Aka Asian liberal democracies (or at least with strong political plurality and civil society). You're mistaking a regional difference with a commonality with the PRC, when in reality the PRC's epistemic worldview is highly distorted in comparison to virtually every other actor in the region. They don't speak for the region.