Maybe make it so that it's all based on modding units to act as those TEC broadcast center thingies. The Rebellion's planets act as broadcast centers, while the Empire's ships act as broadcast centers. Only make it so that the Empire's ships don't spread 'culture' to other planets; they will only bolster the culture on the planet they are orbiting. This way the Rebellion's culture will slowly creep outwards, forcing the Empire to commit more and more resources to fortifying planets. This sounds like such a great idea, and a great way to make the Empire beatable in a strictly canon sense. The Imperial Navy had thousands of Star Destroyers, but in all likelihood, only a fraction of that battle fleet was available for direct combat; the rest undoubtedly was tied up enforcing order on the countless star systems in the Empire.