First of all. Yeah. That makes quite a lot of sense that the poly count costs a lot. With animation it means the system has to do modifications per vertex instead of just rendering. For a mobile device that is quite insane.
You should not try to modify the pathfinding code yourself. Epic has create a quite good system.
But there are a bunch of settings you can play with in the Project settings under “Navigation Mesh”.
Apparently there is also a crowd manager module. Just found that myself so no clue what it does or if it’s useful for you. But crowds sound like something you do.