The notion of city or planetary shields forcing you to besiege multiple planets per system is broadly speaking wrong-headed. Most planets, and even resource zones, have no value. There is no reason to attack targets that already mean nothing to the enemy.
There are two types of planets that matter to war fighting capacity, resource harvesting worlds, and shipyard industry systems. Generally the resource harvesting worlds don't have overmuch effort put into defending them, and if city shields were a concept, resource worlds most likely wouldn't be practical to shield. The majority of the resource extraction nodes could be bombed out, removing the resource world from the equation, unless many separate shields were constructed. This could be limited any number of ways, for instance simply allowing only one shield per world.
There are two types of planets that matter to war fighting capacity, resource harvesting worlds, and shipyard industry systems. Generally the resource harvesting worlds don't have overmuch effort put into defending them, and if city shields were a concept, resource worlds most likely wouldn't be practical to shield. The majority of the resource extraction nodes could be bombed out, removing the resource world from the equation, unless many separate shields were constructed. This could be limited any number of ways, for instance simply allowing only one shield per world.