Hi there,
Its easy enough to have different front and back distances, but per tile would be far more complicated - as it would open up the model and would need sealing up from the X and Y planes. It'll make sense if you imagine 1 1deep tile next to a 3 deep one, the inside face will need filling. It'll be easier to write a pooling collider system that can spawn box colliders dynamically - it'll be a lot easier to maintain and a lot quicker to update too.