Byzantium needs a buff, its been nerfed to the point of unplayability in recent patches, the one strategy that worked reliably was alliance with PLU, but now that does not work on Iron-man as even if you get rivalry bonus to Ottomans, and dip adviser +1 AND give gifts and get marriage, its usually not enough to get Poland to ally you.
It's not usually enough to get Poland to ally you, but it is possible
It requires an insane amount of luck/restarts with alliances/rivalries in Western Europe
What I've done that worked against the Ottomans was an alliance with Poland and Hungrary and declaring war against ottomans while they're distracted/not done fighting in Anatolia