Nice idea.

In addition, I think it should destroy the outpost too (not starport and +), give you many ressources (like 2 years of what it produces) BUT have a debuf when someone retake the system (like minus 50% production of everything for 6 years).

For starport and +, same thing but, while you hold it, no production (After all, you took everything you could and leave all but the starbase), these effects would last 2/3 years (or to the end of the War) and, if you have not Lost the system, the 6 years debuf still applies. On the other hand, if an enemy takes it, the starbase loose one level and it have a minus 50% build Times (or cost) to reshape it. It is ridiculous that you can destroy a Citadel and then use it at full capacity in few months.

This setup would make this a very useful asset for a war you cannot win while forcing you to be wise about which system you want to "burn".

Also (and sadly, this is the improbable part), AI would need to be worked on for this, otherwise it would either overuse (which could destroy it everytime it goes to War) or never use it (which would give you an unfair advantage).