Utopia Strikes Again!!!!!!

This isn't a definite defeat.
He's at civil war, and will likely still be at turn change. If I recall the mechanics correctly it's a 50% chance of rebel reset at each turn change at civil war. If that happens his army size is halved. If we can keep you alive for maybe 3 resets, which should be easier as his army collapses, you can beat him.

If his army is destroyed while you are still at war with him, don't finish him off. Stay at war, attack whenever his troops get above 10k, steal land, gas him when you can, see if you can defeat his airforce and start bombing his factories one by one. Defeat is not the win condition, demoralization is.

Once his military is destroyed we can take ulterior fronts, the first of which will be bribing dsylexia to kill him. Then we can attack from any nearby nation as much as we want. Without his troops he cannot pose any opposition, he cannot attack the rebels, he cannot prevent his annihilation. As soon as he starts taking multiple resets this stops being a meaningful competition, and becomes a game of how to kill him most efficiently.

EDIT: Upon closer research, he needs to reach brink of collapse stability for resets to happen. We have to keep him at high rebels and low approval for several turn changes, while his stability falls. Since he can remove rebels and gain approval whenever he wants using his massive stored wealth, we have to force him to do that so much that he runs out of stored wealth before we can begin to defeat him. This is going to be a long ass campaign.