Efficient use of SC against richer nation

[MaxWilson]

Apologies if this is obvious, but I'm still learning the game (vs the AI still) and I couldn't find this addressed explicitly in a search:

Suppose your research investment into SCs has recently helped you pull ahead on the power curve relative to your larger, richer neighbor who has multiple armies of hundreds of Jotun militia stomping through your territory, and you've decided to go on the offensive and take his territory. Suppose further that he's built a lot of forts and has 6 forts scattered around the 15 provinces closest to your border. What's the most efficient use of your limited number of SCs? Sieging forts seems to take 3-4 turns per fort, which kills momentum, so do you bypass them? If so, how do you hold territory you take? Or do you kill his units in your area first?

I guess it still looks tedious trying to turn this war around, and I was hoping that a good SC would change things more than it has and I'm trying to adapt my military doctrine.

-Max Wilson