How to deal with swarms of 20 stacks?

Picked up Warhammer 3 with the recent sale, but so far all of my campaigns have ended with an enemy faction swarming me with constant 20 stacks of weak units faster than I can replenish or reinforce. For example, I just gave up on a Katarin chaos campaign after I went into the first rifts and immediately had Zhatan declare war and rush my capital with 4 armies. I had 3 armies, Katarin was in the rift and couldn't help in time while the other 2 fought and won against him 3 times only for him to bring two fresh 20 stacks 2 turns later. Even in the rift, the demon prince declared war and lost to Katarin, then showed back up with a stronger, fully healed 20 stack 3 turns later.

I followed guides I read on prioritizing aggressively expanding in the first couple turns then building growth -> income. In the example, I had defensive alliances with all the Empire factions to the south and a non aggression with the Orthodoxy. I just don't understand how I'm supposed to compete with swarms of enemies that seem to respawn in only a couple turns. Does the AI have no upkeep or something? Appreciate any advice anyone can give!

Editing to say thanks for all the advice, I'm gonna try a fresh Kislev run tonight and hopefully do better. Really glad to see such a helpful and welcoming community! Thanks everyone!