How might the Spanish have resolved the problem of finding a dependable labor supply without resorting to the use of slavery?

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.