I really think there is a balanced solution which does not make the bases invulnerable, but won't let hundreds of work hours be destroyed in just 1-2 hours. How about boosting the station's HP or if the repair materials repair a slightly more damage?
I think a good solution would be if a shielded, fully stocked (with 3 repair materials) core4 could be killed by 8-11 battleships in ~4 hours. What do you think? I think that is enough time to get help or proper defenses to save the base.
Can this be done/balanced somehow?