After some thought, I don’t believe the Chinese assumptions on race would be anything like the European. European racial attitudes were shaped by the concentration of Caucasian people into a small geographic area of Europe. Granted there were Caucasoids outside of Europe proper, but generally the populations outside European Christiandom is physically distinct.
The Chinese would see similar looking people across nomadic central Asia and Siberia, Southeast Asia, the Pacific islands, the Americas. Most of these people have little in the way of civilizational achievements the Chinese admire. They would regard the city dwellers of the Middle East and Europe as partially civilized if lesser than their own. But there would not be a catagorical belief that people who look East Asian are better, hence the whole concept of racial superiority.
The first Chinese interaction with sub-Saharan Africans would be with the prosperous Kilwa Empire and that may influence racial perceptions as well.
Literally everything in your post is wrong.
1. No popluations are physically distinct from their neighbors. North Africa, Middle East, and Central Asia all have populations with people who could pass as "looking caucasian" from right across the "border of christendom".
2. Regardless of the distinctiveness of differences between neighboring populations, what really matters are perceived differences. Some perceived differences are easier to notice when you know the two neighboring peoples, but from an outsider perspective "they're all the same to me". So to a European, a Tatar and Han might have "Mongoloid features" but to East Asians, Irish and Polish are indistinguishable.
Furthermore, some perceived differences even between groups on different continents are virtually imaginary, such as the "white" skin color of East Asians and West Europeans. "Yellow Skin" was pretty much invented to give Europeans who were ignorant of Asians a reason to see them as different, other, and thus fearful and untrustworthy.
This leads to -
3. Regardless of differences and similarities, ways will be found to racialize groups based on the political and economic needs of Empire-building. Other Asians with narrow eyes will be seen by the Chinese as inferior regardless of how unclear the dividing line is, just like how Europeans see Middle Easterners as different even though when shown real-life pictures of Syrian people's faces, Americans didn't realize they're weren't European until told so.
4. Civilizational developments don't really matter. They can be explained away, as "Those people couldn't possibly have built those monuments, it was obviously some transient master race which slipped into obscurity from allowing miscegenation with the dirty locals." This was a real attitude towards the achievements of Indians and Africans. Or alternatively the "They were once great but fell into becoming decadent" for the Persians.
Europeans learned of the greatness of Africa's wealth in the Malian and Songhay Empires through Muslim merchants. They did know about Africans having rich and complex societies, but then conveniently forgot about that when it was time to sell them into slavery and colonize them.
Basically, any "evidence" can be subordinated to the political needs of those who wish to exploit others for economic gain.