Perhaps you can make sure your great generals get settled? I don't buy into this heavy reliance on supermedics that so many people seem to love. In the game I mentioned in my previous post, I had 3 settled generals before I could even build maces. I got that many great generals just using archers and longbows in a war with Tokugawa.
With Vassalage my units were already coming out with 11XP. But even with only 1 or 2 generals it was pretty easy, but with Vassalage I admit. With your D2 and D3 longbows though, just pick their battles so they earn the required XP in few battles. eg. if your units are 5XP and you need 10XP for drill IV (Protective), then fight a 2XP and 3XP battle or two 2XP battles and 1 1XP battle to get to 10. It really doesn't take long. Using any drill promoted unit attacking for 1XP is a bit of a waste.
Check out my stack just before I attacked the city.

(see pic) This was Churchill who's Charismatic so the promotions were coming a bit easier but all Protective leaders can more or less do the same thing as this.
Hey at least I'm not going off-topic for once ranting about longbowmen. You are absolutely right about them being very versatile and cheap.