The Spire cities are intended to be the defensive bonus. They can't be the very front line but they can be one system back. Their forcefields haven't blocked movement in the past but you could use them to cover a normal one on the wormhole itself and the result could hold off even a massive attacking force for at least a brief time.
I'm quite happy to buff Spire City defensive modules quite a lot, if that would help. They're supposed to be major starbase-level defenses, etc.
Can also just make their forcefields block movement, to remove the need to add a normal generator to cover the wormhole.
You are using them to cover your wormholes-towards-your-homeworld, right?
Of course, during the buildup, losing one is a problem of its own.