Yea longevity with Greece is tough. I thought that conquering Egypt would be the answer as they have plenty of places to build cottages, unlike Greece, but even with a very early conquest of Egypt I still had a 1 star economy later in the game. This was after I had a very nice game as Japan with 4-5 star economy most of the time.
I had 3 GA's in that game too but like some here I wondered whether that was actually a good thing though it helped me out in the short term obviously, as soon as it was over, like a bad hangover, my stability was worse than before. At the end I took Italy and had culture running at 50% but it was too late.
You really have to place your cities carefully with Greece. The Greek economy rating seems to be similar to the foreign rating you get with Egypt, very difficult to avoid a bad mark here. A bit surprised that my expansion didn't get much higher than 3 stars after absolutely decimating the barbs over and over again. Will try building the economy more slowly and also perhaps building more cities in Greece itself next time.