They could have come up with a paid labor system as well as invest in mechanical that would have revolutionized agriculture.
Breaking down the money that was used to buy slaves, and finance their travel,food and accommodation, one have enough money to pay labor, even though minimum wage for some several years. The problem have to do with the notion of supremacy and savageness that characterized the whole of humanity in that era.
Native Americans most likely built mounds for ceremonial and/or burial purposes. I know this because I am Native American. Good Luck! Hoped this helped.
