There may be some shortcuts depending on the terrain mechanics (like visibility) and the unit behavior.
If visibility is blocked then its unrealistic for a unit to plan out every step of its path to a distant out of visibility (or rough visibility) target.
Getting called (by other units/alarms??) and moving to a known ralleying point near the target would often be done and then finer detail pathfinding from there. You could have a preprocessed ralley point node map (possibly including precalculated stepwise paths between adjacent ralley points). That would be used when the unit is well out of the proximity of the target.
If there are a group of units (you mention hundreds...) ordered to go after the target, they could even share the path calculation and do a simpler 'conga-line' group move
I recall once doing some map analysis to try to build 'zones' on a fine detailed grid map. The map mechanics is important for how you decide what the 'zones' are (I used a group of grid cells that were all visible to each other as well as linear 'tunnel' segments that would also be defined as 'zones'. The set of zones was then used for hierarchical pathing to get units close enough to start using fine grid cell pathfinding.
Depending on the game, you may not even have to have units alwayson the fine terrain grid and instead move them in an abstract way (zone to zone / high level node to high level node)- at least until it was possible for the player/opposing units to see them ---- then they would be 'realized' on the fine grid.for normal interactions/movement.
Oh and if not previously mentioned if you have more open terrain (rooms connected by doors/tunnels) there has been game talk for quite a while about 'portals' - how to find the interface points between zones (often used more for culling out non-visible renderings for terrain/objects in terrain, but may be applicable to (pre)processing sets of 'zones' fro high level movement/pathing.