>I do know they had to practice a fair deal of diplomacy there because they weren’t able to go into the interior w/o dying of disease.

Oh it goes back before the colonisation. Spain's history has been connected to the African Kingdom's back to Carthage. For a long period the Moors ruled Spain. When the Spanish took it back, they kept a large number of their trade routes running.

> I’m guessing one of the first things they did to the Natives was turn them into Christians.

Well somewhat. A lot of the conquistadors were honestly just in it for the money. A lot of natives did convert to the new religion, some by force, some by choice, some just to curry favour.

They likewise were careful to preserve elements of the priest hierarchy in areas they conquered, so they would convert the populace more easily. And expanded a large number of missionaries.

But those mostly came later, at the start a lot of it was just conquest and plunder.