My problem with having ONLY Greece departures available in August (September for some), besides it being HOT, is that there could be a “fleet” of big cruise ships all heading to mostly the same islands. Right now we are booked on Rotterdam 8/22 to Norway.... then on Windstar 9/13 -21 from Athens to Venice. A few weeks ago I checked a port /harbor site for Santorini for our September week and found only one other ship there on our day (9/14) BUT there were other days that same week with multiple cruise ships calling on Santorini . I did read that Santorini has had a specific ceiling each year as to how many cruise ship passengers can visit on any given day.
THIS link lets you see what ships will be in port on specific days. Although this is for Santorini, you can vary and search the same cruise ship schedule for other ports https://www.cruisetimetables.com/cruises-to-santorini-greece.html