I am not qualified to know how this does pathfinding under the hood, but, after hearing that some players are using more than 100k(!) logibots in their bases, it might be a good idea to use their insanely efficient bot waypathing code for mining drones, instead of biter navmesh pathfinding.

Essentially, the station drop-off point and each (unobstructed) cell with ore would serve as waypoints, and drones would pick the shortest (unused) path. If a stationary obstacle is detected, a box of four waypoints just outside of its hitbox would need to be created, and any deposit waypoints that are on the far side of it will have their path expanded to incorporate the obstacle waypoints. (This would only need to be done once, when the mining area is set.)

The bot code would then be used to move each drone along its path to the next waypoint.

The drones should also ignore moving obstacles, as there shouldn't be any. If there are, it is the player's responsibility to replace any mining drones that might get destroyed. (Do you have your drones automatically pick up any ore that drops on the ground within the mining area?)

Does that sound reasonable and doable?