Best way to play Byzantium?

This has become a much harder nation to play with each patch it feels like.The only thing I can figure is to start a game where Serbia does not rival you, royal marry them and hope to get a PU over them since they, like Castille, always disinherit their beginning heir and are like 67 years old.

What other strategies are there? The, get Poland as your ally and use him against the Ottoman doesn't seem to be as helpful/possible as it used to, especially with the whole "Too many diplomatic relations!" modifier.