Well, my character, who is a duke in the CE is quite proud and at times arrogant, and would love nothing more than to be a ruler of his own realm
This directly contradicts this:
There is really no motivation for a Duke of the empire to secede.
No motivation? You just said it up above:
"my character ... would love nothing more than to be a ruler of his own realm". Apparently there is something that your character loves more than being a ruler of his own realm: Being a duke in someone else's realm.