Was there a large population of 'Steppe-influenced Chinese' by the 18th century? Was there, in fact, much of any indication of steppe-culture influence in China by the 19th?
Qing government maintained a very strict control over the migration of Chinese population into Manchuria and Mongolia (then called
Guanwai 關外, "the Land beyond the Passes of the Great Wall") before late 19th century. Technically speaking, the Manchu culture heavily "influenced" the "Chinese" culture (for instance, clothing, language customs, foods), but not to the point of a large scale assimilation.
However, there is one important point about the Manchus - you probably noticed I didn't talk much about Manchus in my last post about assimilation, and this is the reason:
They are not steppe people. They didn't live in the steppes; they were settled farmers with advanced agriculture, and they barely practice pastoral nomadism. Han Chinese and Koreans who lived in Manchuria before 17th century easily assimilated into the Manchus,
because their lifestyle and economical practices were not that different. And, conversely (as you can tell), when Han Chinese migrated into Manchuria in massive numbers in late 19th century, the Manchus assimilated into them as well (but not "easily", it took nearly 100 years and some forced education), for both are agriculturists.
The Mongols, on the other hand, remained on steppes with their own culture and lifestyle. A massive amount of Han Chinese migrated into Mongol territories - on the border of the steppes - since 19th century and turned these regions into farmlands, but the Mongols (even those who now lived in PRC) still retained their culture, social practice, and not to say, language. One of the reason is the Mongols
still practice pastoralism (or at least livestock raising) on the steppes while the Han Chinese are just move into the region for farming and commerce.
That's why I said, after
Humankind's portrayals of the Huns, what 4x games still lacks about steppe people is how they turned a land of a logistic nightmare into an empire based on "highways" - in other words,
how they actually survived in the steppes, while the agriculturists cannot.
The agriculturists cannot simply "transplant" their way of life to the steppes (at least before modern technology such as dams and large-scale irrigation works); if they want to survive, they need to adopt the "steppe way" - and often being assimilated as a result. Conversely, the steppe people (real steppe people, not Manchu farmers), when move into agricultural lands, need to either 1. adopt the agricultural way (being assimilated as a result) 2. turn the agricultural lands into pastures (which is actually quite feasible, Yuan had done this) and continued their pastoralism, but at the cost of reduce agricultural population and their economical outputs in huge amount.
If any 4x games in the future can represent this dynamic - that is, represent the steppes as
unsurvivable for agrarian society (similar to how Desert and Tundra works currently), requires every empire that moved into it to adopt the "
steppe way" in order to survive, and made the steppe feature an important part of these empire's culture traits - then I would safely call it a (nearly) perfect 4x game. That's how the Huns, the Mongols, the Göktürks, etc. made their history.