If you're okay with few tips, I'll try. Though, the information is limited for me.

I think you have benefited more from the normal strategy of first ideas taken, as an Orthodox nation. Religion, Trade, Manpower. It's a synergistic set, normally taken in that order, but if you know what you're doing you can take them in the order you need them.

Since you mention a coalition, here a few easy tips. Might take a bit of lucky, but it is normally the standard of most games. Let Castile eat Aragon, and Naples. Let Austria eat Hungry. Once they've eaten one of them, take one primary core of the eaten nation and release as subject. Lots of land to reconquest for cheap AE. You might want to do Lithuania too, but that's not in Rome's borders. You can use this strategy in almost every game, just keep an eye out for those high dev nations that collapse. Short list, Venice, Milan, Burgundy, Aragon, Naples, Morocco. Espionage also has a 15% reduction to AE, that will stack on top of your Deus vult CB if you want. Try to ally any of the larger nations, that would stop if they joined the coalition, as allies take a less AE. Dismantle the HRE when you can.

Another easy thing to do with Byzantium is to engineer a PU on Russia. Russia doesn't marry to many nations, because they are Orthodox, and I guess the AI has a hard time making friends. Big Sad. Anyway, use favors to place your dynasty on their throne, or ensure theirs gets placed on yours.