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The first obvious target is Dresden, Saxony to your south. Since Saxony is a Polish protectorate this will also spark a war with Poland but it will not drag their allies in which is ideal for you. Take Gdansk next and then you have a consolidated base to build from. There is little to be gained from delaying or making peace with Poland so press on and take Warsaw to break them completely. Make sure you don’t leave Konigsberg too weak because Courland will probably attack from the north.
Now the Austrians are liable to attack you from the south. Keep an army in the south to defend your territory there and build a separate force to march north and wipe out Courland. With Courland gone you can mop up Poland and Lithuania and bring your now veteran troops down to face Austria. Take Prague and Breslau in that order and consolidate your position.
As you progress you’ll need to respond to any raids as fast as possible and make sure the money continues to flow into your coffers. Raiding the towns of your enemies can be a good way to distract them from attacking your towns. Even with small forces that you sacrifice this can be a worthwhile exercise. Make sure you build farms, develop schools and repair the cities you seize.