This response has been heavily edited because I originally got the departure date wrong.
I think this is what you have. I'm showing the minimum number of nights you've specified at each stop; this indicates you have only one extra night to be used somewhere.
Sep 5: Dep from US(?)
Sep 6: Arr Athens and go on to Santorini (2 nights)
Sep 8: Dep Santorini for Milos (2 nights)
Sep 10: Dep Milos to Athens and on to Delphi (1 night)
Sep 11: Dep Delphi to Meteora (1 night)
Sept 12: Dep Meteora to Monodendri or Mikra Papigo (2 nights)
Sep 14: To Athens (2 nights)
One extra night; could be used at your choice of location. The problem is that you probably need one more night at every location, or nearly so.
Sep 17: Dep 6 AM for home
I can't speak from experience since my trips to Greece have primarily involved destinations different from yours, but I think you are far too optimistic about how quickly you can move from place to place and how much you'll be able to see in Greece in such a short time. It usually takes a minimum of half a day every time you change hotels/cities; it can take much longer than that. Spending one night in a city typically gives you about half a day there. If you're tired from the previous day's activities and don't get an early start, you may only have a few hours at your destination.
I don't think one night at Meteora will allow for much hiking after driving up from Delphi, which ViaMichelin estimates will take at least 4-1/2 hours. Those estimates are usually optimistic. They don't allow for stops, traffic slow-ups, or getting lost
ViaMichelin.com estimates the driving time from Monodendri and Mikro Papigo to Athens at between 5 and 6 hours. For that trip you must also allow time to stop for gas, deal with the rental-car return and get from the agency to your hotel. Realistically, you won't be ready to begin your sightseeing in Athens until rather late in the day, and part of the time you do have will presumably be spent a dinner. I'd count that as having only about one full day in Athens for sightseeing. Yet you say you want to spend "a few days" in Athens. To have two days in Athens, you need to spend three nights there.
Some countries in Europe have a lot of express trains that will whisk you between major cities at speeds approaching 200 miles an hour. Greece is not one of those countries (it has few trains of any sort), and in any case you're visiting mostly smaller places. You have only eleven nights in Greece. It's seriously ill-advised to consider dividing that time among six locations. Others may have a different opinion, but I'd recommend Athens if you want to see the classical sites/museums (it's really not otherwise a particularly interesting city), one island (maybe a day-trip to a neighboring island if transportation schedules allow it) or an in-transit visit), and one place where you can do the hiking you want. If you're more about the rural areas than the city, you could spend just your last night in Athens so you'll be near the airport and figure you'll see very little there. That would allow for one island (plus perhaps a day-trip) and two more stops on the mainland or two islands and one stop on the mainland.
People do fairly often make short visits to Santorini because it is both very expensive and very touristy. Otherwise, I'm not a fan of quick trips to Greek islands. I much prefer to have multiple days to settle in and see something other than just the port town. I like to have enough time to know I'm in Greece, which usually means heading inland, away from the beach. Two nights in Santorini followed by two nights on Milos (or Folegandros) would be frustrating to me. I'd be reluctant to spend fewer than three nights on Santorini despite the cost. (Milos I haven't been to.)