After doing some research on Chinese history, I was absolutely astounded at how much land the Han culture expanded into and permantantly assimilated into the Han Chinese culture.
I'm here to ask how they were able to culturally assimilate so much land without succumbing to many of the other problems other powers of the age faced.
That's certainly an... interesting way to spell 'How did the Manchus militarily conquer so much land?' The notion of successful Han Chinese 'assimilation' of frontier regions is frankly a bit of a myth. See this answer on Qing motives for expansion and this answer more specifically on the Qing conquest of Turkestan for a brief overview of how 'China' obtained its modern geographical scope.