It took them several more centuries to conquer the Mayan civilization with New Spain as a base of operations. Without the Aztec conquest giving them that foothold, it would not have played out the same way at all. If the Aztec leaders were determined to slaughter them in the jungle it would have been impassable to the Spaniards regardless of how many cannons, guns, or horses they had. Even before the Conquistadors knew that Tenochtitlan existed, they were almost wiped out several times by tribes on the coast.

Montezuma invited the Conquistadors into the city and they even held him as a hostage for months before any fighting broke out. Those alliances took a while to establish and the germs only came later into the campaign when they started sending ships back for more troops during a Caribbean smallpox outbreak.

I recommend reading Conquest by Hugh Thomas. The whole story is fascinating and very nuanced - it took several expeditions to even get that far and Cortez was possibly the luckiest man alive in the 16th century.