Well, according to the charts, *scouts* are actually the best at taking out siege (and LRFs). Bet nobody tried that yet! So perhaps leave some supply room, or have a small fleet of scouts ready for defense.
The whole idea of the "front line" in space makes ZERO sense. I agree with boxox, making PJIs more effective has to been carefully done. A little more, ok; a lot stronger, and it becomes a "battle of the choke points".
If that happens, people will mass their fleets even worse, and cripple any hit 'n' run-type tactics. It becomes just another "throw everything into one big fight," like every other RTS.

P.S. If you think games take long now, increasing planetary defense strength will just make things worse, IMHO.