Just pre-compute everything. Put your vector fields in textures at map design time and be done with it. No point in overthinking this once its a simple lookup per frame.

The real fun is designing the agent's AI: exploration, avoidance, engagement ;)

Crowd Pathfinding And Steering Using Flow Field Tiles

http://www.gameaipro.com/GameAIPro/GameAIPro_Chapter23_Crowd...

Flow Field Pathfinding Tutorial

http://leifnode.com/2013/12/flow-field-pathfinding/

I think you're forgetting there is no map design time in Age of Empires since the maps are RNG. Also, walls, structures, and destroyed forests are critical to gameplay, so it has to be an extremely dynamic process.

There are obviously ways to improve the pathfinding, https://github.com/SFTtech/openage has many plans, but your dismissals are starting from wrong assumptions.