What if we add 3rd dimension?
How will it work:
= Game will have 3 layers.
= Ship is on 1st layer has scale x1.5, other that is on 2nd x1 and next one is on 3rd has x0.5. (for example)
= Ships that at different layers cannot make a collision. The model of upper will cover other ship.
= When you are moving camera, perspective is moving ships on the layers proportionally.
= Ships can miss not only in one surface, but a sheel can fly to other layer (If each ship has a hisself layers, then a shell can fly to the tower directly (towers will place above whole ship)).
Here is an example of perspective. Imagine that all ships has same sizes, but they are on different layers. Also, on perimeter ships are on place. If u will move ur camera there, upper one will cover others:

I don't know how will it laggy, although as an idea... why not?