You all have a very different idea then me on what Aztecs are infact for.

Give a very hefty bribe to improve your relations to perfect, make an Alliance, gift them some of your enemies cities in the old world, and laugh as your Aztec allies make mincemeat out of your enemies

You need to bring entire stacks of cavalry along endless oceans, and they can't be replenished in the new world, the mercenaries are very expensive and limited in number, and there aren't enough cities to seriously effect wether you win the campaign or not, and it is funny seeing your enemies falling to cotton wearing club wielders

I tried bringing Noble Cavalry, Lancers, and French Mounted Archers before, your casualties will be very high, the all noble army you bring hurts realism, they will win by right of their large numbers of soldiers, and you will have a massive drain on time better spent fighting your true enemies.

Last but not least as a good reason to ally the aztecs, Gold, gold, chocolate, tobbacco, chocalate, and chocolate, the new world is full of resources even a 1 star finance merchant will rake in money from, and it's better to bring the merchants then the cavalry, especially since merchants are free.

Which is the reason I never even try to conquer the Aztecs