Ended up mopping the floor with both Milan and Aragon, largely thanks to the naval efforts of Venice.
Milan wasn't too hard; they had just had a huge battle with Venice. Though they had won against Venice, that battle had made it easy to break their morale and sweep up the leftover routed forces. I set up sieges across Milan, and recalled my forces to the 'boot of Italy' to beat back a few thousand Aragonese soldiers that had come over to invade after being chased off of Sicily by rebels.
During the war, those same Sicilian rebels ended up causing Aragon to release Pelermo into a new Sicily. The rebels didn't have control of Messina because I decided to, after eradicating the Aragonese incursion, get rid of the 18k stack while Aragon was busy with Venice. So I saddled up the Reconquest Casus Belli, took Palermo, and annexed Sicily pretty swiftly. Messina fell from Aragonese control soon afterwards.
My initial invasion of mainland Aragon was assisted by my new alliance with Castille, who had also agreed to give me military access. I didn't bother sending them a CTA, but being able to land on their shores to sweep into Aragon helped immensely. Truth be told, I likely could have entered Aragon by way of an undefended coastal province as Venice was just starting to land troops in North-Eastern Aragon and they were preoccupied there.
I only met brief resistance from stragglers who hadn't been sent to defend against the Venetian front, and after a mere three battles, I was besieging every province in Aragon that Venice hadn't sunk its claws into.
I kept getting spammed with attempts at peace treaties the entire time, mostly from Milan. I didn't want to accept their peace treaties prematurely simply because I wanted to get some territory from Aragon first, and Milan was the senior partner. I noticed being the senior partner in a war is quite a nice place to be though, from a player's point of view, as the AI can be outright dumb with diplomacy.
In the end, I took Cremona, Parma, and Brescia from Milan, and Valencia, Messina, and Alicante from Aragon. I also had Aragon release Sardinia and pay me 25 ducats in tribute, and had Milan annul its treaties with Aragon. So between numerous war subsidies from Castille and England and some profitable new territory, I'm rolling in the money. I'm aiming to increase my navy and land forces by a fair amount. I did accrue quite a bit of BB, but I'm not too worried. Castille drew me into a war with the Mamluks, but I'm contemplating not actually doing anything. I may raid them a little and settle for a White Peace or some tribute in ducats.
Oh, and another thing; I managed to finally get Tuscany to agree to become my vassal! Now I'm working on Sardinia and Ferrara. Whoever said violence is never the solution has clearly never played EU3.