I was never quite sure what, if anything, my air defenses were doing. Maybe they have shorter ranges than I thought.
I suggest looking to a kind of old game, "Moonbase Commander" for some inspiration on how air defenses could work. Essentially, anti-air is powerful, but can only fire sometimes, they need to first be neutralized with cluster bombs first so that they use up their ammo and then you can destroy them with powerful rockets or bypass them altogether.