Hm.. not sure if home planet invulnerability is the way to go. Nothing would stop a big player from simply taking over the other planets, and parking a huge armada near the home planet, waiting out the 30 days and then taking it anyway.
The planetary defense would work, I think. It's expensive, though, so perhaps make it free for a period of 14-30 days, or make the cost gradually increase over that time?
