Easy way to upgrade the AI

It's hard to build a good game AI. And it's not something you're going to rebuild on the spot. But there are a few ways to make the existing AI play more effectively and provide a greater challenge to a human opponent.

Limit the options the AI can choose from.

Buildings. No air bases, no infra, no more than 2 types of unit factories per city, limit the damage the AI can do by randomly picking the next city or rural improvement.

Units. No air force, no carriers (seriously, this just happened, the AI started spamming carriers). Focus on stealth units like subs, AT guns, AA guns, and simple units like infantry and artillery.

That kind of AI would be a lot more effective than one allowed to spam random junk.