I would move the hull to where the baloon was (Next to the stairs on the upper deck). That way the spire will be tanky in the sense if focused.
This would be a significant buff and definitely worth considering if an armor increase is deemed unbalanced.
My general sense is that the Spire shouldn't necessarily be "tanky" but it needs more durability than it has now so that it can leverage its firepower for a longer duration. Moving the hull closer to the guns will allow a quick mallet or chem spray in effect extending the armor up period and allowing more shots to go out before rebuild is required and 1-2 guns are no longer shooting.