Interesting, as always with your projects.
Quite a few years ago the club I was in decided to do Conquistadors.
I read Diaz, & also heartily recommend it.
We went down the line of using the, then new, Hordes of the Things rules,
(Fantasy DBA).
Horde troop type solved the problem of Aztec armies seemingly endless
supply of poor quality infantry. If they are killed you can bring them back
on your base line as the 2nd, 3rd, etc. wave.
Another problem was the Aztec over-riding desire to take casualties in
preference to just killing Spaniards. This we solved by a rule tweak.
An Aztec unit that won a melee took prisoners & pushed the Spanish base
back. How to keep track on table was my idea. We made abacus like skull
racks. These looked nice on the table, & were a doddle to use.
If you are interested I will try & find the QRS we used & send you a copy.