I agree with the notion that Han (Huaxia or whatever name) as a culture identity is founded somewhere 3000 years ago by Zhou and Qin. But if I understand you right, you seem to downplay the continous infusing of "foreign/borderland" people into Han people since then.
The trueth is that, the infusing is not only continuing but in even larger scale. Just list a few ethnic groups that disapeared since Qin dynasty some 2000 years ago, Xiongnu, Xianbei, Southern Turk, Khitan, Jurchen, Dangxiang, Jie (羯), Di (氐), shatuo (沙陀). They all amounted large percentage when migrated into "China proper" then totally disapeared. Even the word "Han people" is created by Mongol Yuan dynasy in 13th centry to include anyone speaking Mandarian dialects in northern China that includes Jurchen and Khitan which ironically is genetically and linguistically Mongolic people. The succeeding Ming dynasty continued that naming by further inclusion of the Mongol troops that switched side to Ming. Think about the Tang and Sui expresses and Prime ministers and many generals are ethnic Xianbei, Tujue (turk), that shows how big the scale of ethnic infusing was in 400 to 500AD.
This kind of infusing of large number of non-Han continued in southern China as well after Qin and no smaller than the north. Many today's southern Han are ethnic minorities in large numbers who switched name and language. Example is the Bozhou Yang family that ruled part of SW China through Yuan to Ming claiming of being Han from Hongnong county of Shaanxi, but in reality they are local Miao people.
I am trying to say that there is no clear cut historical boundry when Han is made/fixed (culturally or genetically) then remained relative stable and distinct, but it is a continous and increasingly larger snowballing process.
