The TL Land of Sweetness had a Mayan conquerer who claimed himself Kulkulkan/Quetzalcoatl returned who forbade human sacrifice and demanded that the people only sacrifice hummingbirds, butterflies, and snakes. This IIRC reflects eschatological beliefs of how things would be when the Feathered Serpent returned. So even if it didn't play out like in Land of Sweetness, it's perhaps possible for something similar to happen to abolish human sacrifice.
The biggest problem they face is the imminent implosion of their society due to a few decades of repeated epidemics as European disease establishes itself. Which might give a good reason to convert to Christianity, but not to submit to Iberian control without a fight. There's a dynamic similar to Sengoku-era Japan here in terms of access to European weapons (and horses).
What agricultural techniques? The Mesoamericans were pretty successful as it was and the biggest improvement they'd need is adding European domestic animals, which they certainly have the gold and silver to pay for.Christianity combinend with Agricultural techniques. Afther a couple of skirmishes and the potential to introduce horses and guns especialy a monotheistic culture would accept christianity. The aztecs could read and write in their own letters, they might send a message to the pope in rome delivered by rivals of spain (most probably portugal. Paying a certain amount of silver to have the letters delivered and to recieve a response would probably do the trick). If the popes writes a bull that christians can't be enslaved. Also this would be afther the treaty of tordesillas. France could use it as a pretext to call the treaty void.
The biggest problem they face is the imminent implosion of their society due to a few decades of repeated epidemics as European disease establishes itself. Which might give a good reason to convert to Christianity, but not to submit to Iberian control without a fight. There's a dynamic similar to Sengoku-era Japan here in terms of access to European weapons (and horses).