Also there is only one defensive platform
Sounds like you have a badly bugged version!
As TEC you should have Gauss Cannons, Hangars, repair centers, Phase Jump Inhibitors, and a shield generator.
The Vasari get Phase Missile platforms, hangars, repair centers, PJI's, and a weapons Jammer.
Properly used, while these won't
prevent an enemy from destroying a planet, they
will seriously delay him, leaving you time to counterattack, or reinforce your defenses. The only major "hole" in the defenses is flak -- the only anti-fighter ability you have comes from fighters, which means your defense can get shot down rather easily, leaving you vulnerable. Just build some flak frigates and put them on hold position.