Yeah, I'd imagine you'd always split this into two separate problems of pathfinding and crowd simulation. Making thousands of separate calls to a pathfinding algorithm that returns the same path (or very close to it) would be mayhem for a game like Total War. Would be super interesting to read through some of the original source for that from Total War.
Short video on a basic sort of crowd simulation would work in Unity: https://learn.unity.com/tutorial/moving-as-one