My first choice would be to get March of the Damned instead of three battlepacks. It has good cards for all factions (least so for the Orcs, but they benefited the most from the battlepacks you already have), and if you're concerned about balance, its neutral cards can easily serve to close the gap for whatever factions you find lacking. The Legends expansion might also be better than three battlepacks, but since you're already introducing two new factions with Assault on Ulthuan, you might not want a new cardtype just yet.
As for battlepacks, finishing the corruption cycle should be of low priority. It doesn't have three copies of every card, and you don't care much about skaven. Keep in mind there's no reason to buy battlepacks in order though, so you don't have to go with the first three of the enemy cycle, and it might go counter to what you're trying to accomplish. For example, Empire and Dwarf will benefit much more from both The Burning of Derricksburg and The Fall of Karak Grimaz than the other factions, so you shouldn't get those if they are already doing well in your environment.
I'd either go with March of the Damned, or just get Assault for now and wait a bit to see which factions need the most help, and ask again then (or check battlepack/expansion contents yourself on deckbox.org) and pick buys accordingly.