Considering your interests (great food, hiking, beaches, nightlife unimportant) then Naxos should definitely be your other island. (And Naxos has many daily ferry connections with both Santorini and Athens.) Also, Athens needs at least one full day to explore so you should drop any thoughts about Delphi or Nafplio. Also, I would look into flights from Athens to Santorini on your night of arrival. If you could get to Santorini that night (and move your day in Athens to the end of your trip) you’d almost gain an entire day and could spend two nights on Naxos.
I’m a little worried I’ll get bored in Milos. My husband likes to stay in one place, but I like exploring. We would have split our time more evenly but we want to stay on the caldera and it’s just too expensive to stay any longer than two or three nights. Alternatively, we could stay in Santorini for longer but move to a cheaper hotel. Perhaps Santorini 6 nights and Milos 6 nights?