Yes, I'm pretty sure we all know that The White-Gold Concordat very much weakened the Empire.

We also know that it caused the Stormcloak Rebellion.

But Let's review some facts.

Concordat signed.
Hammerfell secedes.
Torryg killed and Rebellion starts.

However, Ulfric was at some point prisoner to the Dominion during the Great War.
Is it possible that it was his idea to ban Talos worship in order to weaken the Empire by making Skyrim secede?

Was the Markarth Incident scheduled to weaken the Reach from an attack from Hammerfell, which the Dominion wanted?

If we assume this, Skyrim( If the Stormcloaks win) is essentially another Thalmor puppet?

By Skyrim seceding, the Empire would be cut off frrom High Rock, which would likely secede or succumb to Thalmor invasion from Skyrim.

The Empire would then consist of a weakened Cyrodiil and part of a devastated Morrowind.

The Concordat was all a vast conspiracy...

The WGC, including the Talos worship ban, was already foisted upon the Empire in a prototypical form before the Great War ever started, in the form of the "peace treaty" that accompanied the big box full of Blades heads.

The capture of Ulfric was a windfall, and his subsequent psychological compromising by them enabled their plans further, but Ulfric had no part in the initial plans to schism the Empire.

This. The Thalmor got what they wanted, even if they had to win it in a war.