is the AI cheating

[thejeff]

The standard advice is to boost independent strength and leave the rest pretty much the same. Boosting sites might make sense, the AI doesn't tend to site search well or make good use of the gems it does find.

Boosting supplies definitely helps the AI. Less of those hordes will be starving. You can work with smaller, more elite armies or make use of supply items to avoid starving.

And I would never waste anywhere near that much money on PD. Maybe in crucial chokepoints, backed up with some mage support, but not in every province. In most cases even 75 PD won't stop a 400/500 AI chaff, and if you're able to attack and expand that huge investment sits useless in your backfield.
PD and troops on your borders do discourage the AI from declaring war in the first place though, as does your Army Strength rating (graph).

I found I usually did better with more room to expand before running into an enemy. Maps with more provinces per nations helped. Gave me time to get the early research tactics in place.

Play around with different nations and tactics. Maybe what you've been using just isn't suited to your style.