[x] Plan North: You're not confident in Schwarster's positions, and more importantly you're concerned by the fact he's only reported sporadic enemy contact. If you can work out a way to extend his supply area into the highlands proper, you can press the enemy dearly from a secure location in the massive highlands around the Great Lake.
[X] Plan South: You're panicking at the disaster of supply that is Holn's army. Three rifle calibers, dozens of makes and model of tractor and boat, and if reports are correct he's got more types of main battery artillery than the Irromic Empire's used since it's formation. If you can clean house behind his operation, he'll be in a prime position to advance and seize ground, forcing the enemy into a limited-frontage battle your armies spent the last six years turning into an art.
[x] Get Schwarster to stick to small unit operations, and work on extending supply lines through road improvements or whatever means we have to improve the transportation network to the Great Lake, and then supply by sea or something until we can figure out land routes.
[x] See what he'd need to start running search and destroy operations, or at least burn us a big enough buffer that large enemy formations won't be able to live off the land.