I only see a slight problem with the Data Center solution and that is that while your total AIP doesn't change, your floor will go up. Otherwise it's a nice design.
A rising floor would be a penalty for losing the system if it is an AI that recaps the planet.
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I am starting to lean in the direction of a new "Minor Faction"
If the "recap" idea is incorporated as a Minor Faction -- Recap Golem with suporting fleet -- that could be effective as a substitue for "a more aggressive AI" with out changing the basic AI coding or incuring an increase in the AI progress/floor.
Recap Golems could show up every few hours of game time, or with some other game trigger. The Recap Golem should be resonably hard to kill (similar to a SuperFort with a strong anti FF buff) have a supporting fleet and take maybe 10 minutes to begin building additional defenses after killing the human Com center.
It may have a special unit or tactic that makes it hard to bypass with transports.
It should be able to convert any left over in-system AI structures (mines, turrets, etc) for its own use. This could apply to human owned structures and capturables (core factories etc.) as well.
It may reward the player that defeats it with a ship unlock similar to the saved rebel human colonies tech.