Im no expert but dont see how changing to a different grinder would make much difference.
Those dose levels certainly are high, your single is enough for a double. Youre effectively serving up doppio ristretto bases to your customers (I wouldnt be complaining if I were one of them).
Couldnt you just go a shallower basket and adjust your grind? Obviously you dont want to sacrifice the quality of your coffee but at the moment you must be spending ~$0.50 per single shot depending on what your beans are costing (plus cup/lid and milk).