If you want to do the ruler designer way, I'd suggest you start the game by typing in ''event 55001'', which will make the province you start in your culture and religion, as else you'd probably get crushed by a liberation revolt of 50000 men from the same county a few years after you start (Believe me, I've tried). This also allows you to circumvent the issue with them not arriving before 1250, if you want to play Aztecs earlier.
It truly revamps the way each game and ruler. Way of Life allows you to add a Focus for your characters. Contains: 'Ancient Religions Reborn' 'Another Traits Mod II' 'Aztec Dynasty Flags' 'Bloodline Fix' 'Born This Way' 'Bunch of Bloodlines' 'Caesar's Books' 'Caesar's Wonders' 'Cities of Wonders' 'CK2RPG Burn Holdings' 'CK2RPG Siege Victory Choices' 'Color Galore' 'Combat Rebalance' 'Cultural Emergence' 'Cultures+' 'Easy Heresy' Religions Added 'Enatic Kingdoms' 'Extra Bloodlines 1.4' 'Friend. You could also use the console and type in ''event SSI.1'' to trigger the event chain that should eventually lead to them arriving, if you just want to do it as a one-off thing. This expansion Crusader Kings II add-on offers the most bang for the buck. It's currently set to 720 Months, which is fairly high, by lowering it to 1 month you'll essentially guarantee it happening. It's very possible to not have the Aztecs invade at all due to bad (Or good) luck, as they won't come once the year is past 1349, and by reducing the MTTH, you drastically increase the chances of it triggering.
Rather than changing the trigger date, you should change the MTTH, or mean_time_to_happen.
