I understand that you put a probabilty check in, which checks EVERY system vs the 1/600 "odds", right ?
But that means if you have like 10 colonies the total probability ONE such event occurs raises by a factor of 10 !
The more colonies you have - the more chance you have.
Thus probably resulting in my experience of "this shit happens WAaaay too often".
If you intention is to have such an event trigger ONCE per empire ~500 Turns...you cannot allow the check to be done per colony.
OK nice it wont hit a system twice - but if it hits another system with in 30 turns - its WAY to strong. I remember I had my first game running until turn 250 around and had at least 5 system deteriorating events getting triggered in that timescale.I made it 600 turns actually And they will not hit a system twice. Supernova can only be triggered once per civ. And they all have only a 50% chance to happen.
Maybe even more than 5...too long ago. But I remember 5 of them. And especially there were 2 events just immediately after each other (like in Turn 199 and turn 200 or so).