I actually found Venice really easy, but maybe that was because I never had to go to war against the Ottomans. Periodically sending a diplomat over there to raise relations helped me keep my relationship with them in the positives. Also, Austria seemed to self-destruct fairly quickly. After about 10 years into the game, while I was busy coring Naples, I saw that a succession of wars and rebels had driven them down to less than 8000 troops. Bavaria and I partitioned them pretty quick, and then there was nothing to stop me from gobbing up the rest of northern Italy.
So as long as you're able to keep the Emperor weak and appease the Ottomans, it should be smooth sailing. I could see how things would get hairy otherwise.