For a moment there, I thought extending the war meant attacking the other Electors to dismantle the Empire. You seemed well on your way: two (Brandenburg and Pommern) are Bohemian vassals involved in that war, Styria came in with Finland - but you let that last one go and the new war doesn't bring any new ones in. An opportunity like this one, half of Bohemia and most of its vassals already occupied - go for it, I say, go for it!