Starbases as they are, have laughably short ranges and generally can do nothing to stop fleets from just passing them by. They are particularly useless in uninhabilable gravity wells like stars or gas giants where they have few options to back them up. I think giving starbases an option for a phase jump inhhibitor-like upgrade, along with an overall increase in range, would go a long way from keeping invading forces in line. Perhaps even make it an antimatter-using ability to keep it balanced, but as it is right now, there is no reason to engage starbases, particularly in uninhabiable gravity wells around stars or gas giants.