How long did the Invasion last? Especially the initial ground force landings and surrender, and the delay between laser bombardment and planetary invasion.
Every 8h they dont surrender is 8h where combat inflicts collateral damage. Even if you are pure PWI, their bombardment weapons will (afaik) still inflict it.
The invasion lasted 40 hours or 5 ground combat turns, and the only thing I used was CAP. I'm guessing their bombardment weapons were the main culprit. They had a couple thousand medium tanks and artillery.
The dust shouldn't be a problem at that amount; Anti-Greenhouse Factor = 1 + (Dust Level / 20000) + Pressure of Anti-Greenhouse Gases. There certainly isn't enough dust to lower the temp enough to turn a cc 0 world to cc 1. 5. And since it is a flat 1. 5, and the atmo on that summary shows Not Breathable, it's very likely the world spawned with a dangerous gas and the OP has cc reduction tech researched to 25%. (I'm pretty confident in this since the OP has said the 19m pop world with the same species is also cc 1. 5, but the OP could confirm this by checking what gasses are in the atmo of the conquered home world. ) This would cause a mass die-off of the spawned population before it built enough infrastructure for the survivors.
There is 32% oxygen in the atmosphere which is higher than the 30% limit, so that's 2. 00 CC, and factoring in tech that becomes 1. 50 CC.
But I'm glad you brought up the fact that dust settles. While I doubt the OP waited an in-game decade to ask about this (it's status is still Conquered), that image might not be the first increment after conquest, and some would have settled during the ground war. The OP calculated casualties of 4. 68m based on the current dust; every 1 dust that settled is another 40k.
The screenshot is right after the end of the combat which lasted 40 hours, so the dust level shouldn't have settled too much.
