In October 2018 I took a similar solo trip. My trip was 14 nights, one night on the plane, 13 nights on the ground. I started in Nafplio (flights from Detroit to Athens, buses to Nafplio), 3 nights in Nafplio. Maybe with a 4th night I could have taken another day trip. I took buses to Epidavros and then Mycenae both the same day. From Mycenae I had to take a taxi from there to the bus in the parking lot of a cafe because the bus doesn't leave from Mycenae in the off season. Then i spent 2 nights on Crete. I suppose I would have been as happy if I had went to Olympia, the most well known of 4 sites of ancient Olympic games, in Western Greece.
In Crete, are you sure you want to spent 4 nights in Chania or would you like to split your time between Heraklion and Chania? I was in both. Heraklion is rather underrated, maybe because I saw a retaining wall instead of a beach, and much of it was destroyed in world war 2. Heraklion has an archaeology museum that might have occupied me for 3 hours, the Knossos Minoan site, a Venetian fortress called Koules that has been marvelously restored and you can pay to go inside, parts of old city walls, a reconstructed Venetian looking town hall, and so on. Check the Rough Guide to Crete, and/or the Lonely Planet guide to Crete. With more time on Crete I would have seen Rethymno, the Samaria gorge, and possibly somewhere else.
I spent t a night in Delphi, the night after my ferry from Chania to Piraeus. A night in Delphi is a good plan. There isn't necessarily anything wrong with going to Delphi after Nafplio. On the way you would just switch buses in Athens. You might have to buy just a ticket to Athens and then in Athens buy another ticket to Delphi. I took overnight ferries from Piraeus to Heraklio. And from Chania to Piraeus. The ferries worked ok. I had a bed a in shared room with 4 beds both ways. One way i had the room to myself. The other way there was just me and a non-english speaking guy who stayed quite the vast majority of the time. Maybe I would have seen better scenery from a plane.
Are you sure 3 nights is enough for Athens?