Well, that's a tricky one for me to answer, but I'll give it a go.
Greece is a lovely destination, I agree, but Turkey is quite mind-blowing. My first reaction would be to advise you to split your time between them 50/50. However, you're in the driving seat, so, an alternative from out or in from/to Istanbul would be to get a flight (in or out) from/to Izmir from where Pegasus
have flights to/from Athens.
There are many flights with several airlines from Istanbul to Izmir, and then you could visit Ephesus at your leisure.
I'm not sure exactly what I'm advising, but I guess it's - fly into Istanbul, look around the city for a few days, fly down to Izmir, look around that area for a few days, fly to Athens, visit Greece, fly from Athens back to Istanbul. Or the other way round.
Something like that?