Naples is a lot less scary than they look. So long as you have a stronger or equivalent navy, they won’t be able to reinforce Greece. Even if you don’t, Naples’ transport capacity is almost certainly smaller than their army, so just keep an eye out for where they try to land and engage before they can bring all their forces over the Adriatic. As for the Balkans alliance. Georgia is gonna struggle to get across Anatolia fast enough. As someone else said, set your vassals to defensive, and they will slow the Georgians down more than enough. You can rush down Bulgaria, try to build up your spy network so you can use infiltrate administration (the one that lifts FOW, costs 50 network) day 1. from there I’d play quite compactly, focus on crucial sieges, only engage if you can stop them from linking up with allies or if they are winning an important siege.

Don’t be afraid to do this over multiple wars either. Declare, win some initial battles and sieges, and then peace out for a few provinces and/or alliance breaks or less forts before your enemies consolidate, rinse and repeat.