In OTL, the Polish King Wladyslaw IV came up with a plan for a war against Turkey over the west coast of the Black Sea. The Senate voted the war down. Such a war would have involved battle ready Cossacks in plunder of Turkish lands and perhaps unified the Commonwealth against an external enemy. Could the Poles-Lithuanians-Cossacks have marched as far south as Bulgaria? Some Polish magnates did have dreams of a Crusade to liberate Constantinople...
Yeah, I'm agreeing with what's already been said.
-Why would this happen? What has changed the attitude of the Szlachta?
-Why do the Turks just curl up and die? In the 17th century, it was the Ottomans who were getting the better of Poland in their confrontations.
-How is it "liberation" to impose Christian Polish-Cossack rule on the Muslim people of the Crimea? Hell, even the Orthodox Bulgarians are unlikely to welcome the "liberators", let alone the population of the large towns which was, in the 17th century, mostly Muslim, I believe.