I wonder if you couldn't tie truce length to power tier. So, (numbers just as a start) Great Powers have a base 4-year truce duration, major = 3 yrs, minor/unrecognized major = 2 yrs, insignificant/unrecognized minor = 1 yr, use the lower of the two as the truce length for a given war.

I dunno if the AI can be made to take advantage of the changes, but at least in theory, the idea is great powers are encouraged to go big and throw haymakers when they go to war with each other, with the truce both preventing you from efficiently taking small chunks at a time, but also protecting you in the aftermath of a big war as you recover. Meanwhile, nipping parts off of lesser powers becomes faster the weaker they are, but also, if they feel froggy, the small power might run off and come back after the short truce with a big friend that they buttered up.

Taken further, apply modifiers based on circumstances, such as relative rank difference, so if two equal powers go to war, it adds an extra year to the truce (giving england vs france a default 5-year truce). Also, capitulation reduces truce length by 33-50% or so, under the theory that appeasement discourages no one.