The way AI fights wars really ruins the late game for me especially if you're a large country like Russia. Even if the fight is even, unless they have overwhelming numbers they'll just run across the world to start sieging you in Siberia without a worry in the world while you siege down their homeland where presumably all their families live without even making an attempt to defend.
It just makes absolutely no sense how these armies could sustain themselves in the frozen wastelands of Siberia after traveling halfway around the world through hostile territory and as soon as you walk over to fight them you can be sure that they'll sense you coming and begin their next marathon to head back home or to whatever one province hole you've got somewhere, there's just no strategic value in it at all for them - it doesn't matter if they occupy a bunch of wastelands at the far extremities of my land if they lose their entire country. Late game wars just feel like playing chase rather than fighting a war
the AI should fight hopelessly against you, it should do whatever is necessary to defend it's home and allied territory. It's playstyle is so predictable that you can sometimes just conquer entire territories without a single fight because the AI is so desperate to avoid battle