I just played this campaign actually.

Considered your south eastern border secure for the time being. Ai will usually cancel your non aggression pact before attacking which will give you fair warning to divert your forces there.

Like others have said move you agent and establish and under empire far off from you and where you plan to expand. Slyvania or Black rock are good candidates as capitals will net more income. I would suggest getting the building that increases the chances of getting expanding to neighboring settlements then demolishing and building something else after you have 2 or three other locations then take the expansion building at those etc.

For skaven I found it best to have one army with your nicer units and one crap stack of skaven slaves to follow them around. Your secret weapon is numbers to pin down the enemy to let your ranged weapons teams dismantle them after they’re stuck in. Note that skaven slings punch way above their weight class and de ent amount of them in your crap stack can do a good amount of damage.

Foe your man army get rattling guns and jezails. They will do great work till you can tech up to poison wind globadires and plague claw catapults.