Hello, from what I understand, the inactive player is a problem.
I thought of a few possible solution:
The solution: radical: if the player is inactif for over 3/6 months == self destruction
The solution collaborative: an inactive player for a while to find a list of contact person to clean by SirParadox on its website
The solution deterrent: an inactive player for some time is left without guild without trade income without alias
I think inactive players should lose their invulnerability after 3 months of inactivity then automatically self destruct after 6 months. I've noticed some seem to disappear after awhile, but it seems it takes some time.
Another idea would be for the inactive player colonies to be taken over by pirates and become pirate colonies after 6 months. Pirate colonies generate units within their command points limit until destroyed. Generated units remain within the environment they spawned in and attack whatever is there.