The Sorcerer King needs some sort of mechanism in the final quarter or so of the game on par with the "Realm Divide" of Shogun 2 or the Civil War of Shogun 2: Fall of the Samurai.

Right now it's like this (from the SK's point of view):

Part 1: Oh, I've won. This guy is just a meek little administrator serving me.

Part 2; This guy is actually standing up to me and building up his armies.

Part 3: Now this guy is a threat - he's actually killing my senior commanders.

Part 4: Holy crap! This guy might actually win! I gotta throw everything I have at him!

It's the Part 4 that's a bit underwhelming. The SK is supposed to be immensely powerful. Part 4 should mean that the SK is summoning everything he's got and hitting you really, really hard.  Plus at higher difficulty levels, the enemy has massive numbers.

One thing I would recommend is making Part 4 more "unique". One of the things I will note about always fighting the SK is that unlike other factions in FE LH, the SK's tactics are too standardized. It should be different each time.