So when the pope commands me to join the crusades, I say YES SIR like any good Christian king…but only just before mission expiry.
Then I’ll let my expensive units join, and suddenly I’m awash with cash cos my army upkeep is so low.
I’ll spend all the money on big building projects, while keeping the balance just below 20,000. I recruit as many 4 exp crusader knights and horses as I can find.
To keep the crusading army’s dream alive, I make tiny movements towards the target, sometimes switching units in and out of settlements just to keep them from deserting.

Usually by the time the crusades end, I’m still near to my regions, and I can quickly pull back with my shiny new exceptional units with lower upkeep costs.
My bank balance is healthy, and my settlements get a construction boost.

Not historical, but hey it’s all about selling a dream to the crusaders.