Fast way: Declare war and sponsor rebels all over the place. Occupy some stuff fast (and the unconnected stuff ofc), then just defend and let the WE and the rebels kill them. May require a lot of minting.
Long way: Conquer a land connection to your Bulgarian holdings (via Ukraine and Moldavia), then go east. Anatolia is pretty rich, so are parts of Russia. Conquer from there further in any direction, you're still Austria and therefore pretty strong. You could also colonize or conquer/inherit the western parts of Europe. Once you're strong enough, kill that pesky HRE.
Long way: Conquer a land connection to your Bulgarian holdings (via Ukraine and Moldavia), then go east. Anatolia is pretty rich, so are parts of Russia. Conquer from there further in any direction, you're still Austria and therefore pretty strong. You could also colonize or conquer/inherit the western parts of Europe. Once you're strong enough, kill that pesky HRE.