I've keep greek for historical and weepy reason, but it was a bad idea, for some meager income i have to spend a lot to protect it, and wasn't able to avoid raiding there. Keeping Constantinople and anatolia you should be able to defend your west front with only two small armys. The first thing's i advice you to do is to give up the european side of your empire (Greek Makedonia and Thracian) or at least the town with no walls in the north.
