Once you add in all that stuff I think the multiple Hull idea may be cheaper.
What about a new thing, streamlined external cargo mounts? +20% cost to the external cargo mount, maybe with a -20% capacity penalty as well (regular internal cargo on a streamlined ship doesn't run into that as you can use fuel tanks, electronics and other components to fill the awkward spaces created by streamlining)? Still subject to that landing penalty, but avoids the normal rolls for atmospheric operations.
What about a new thing, streamlined external cargo mounts? +20% cost to the external cargo mount, maybe with a -20% capacity penalty as well (regular internal cargo on a streamlined ship doesn't run into that as you can use fuel tanks, electronics and other components to fill the awkward spaces created by streamlining)? Still subject to that landing penalty, but avoids the normal rolls for atmospheric operations.