I think this is a wonderful idea! I agree, save Athens for the last, for 2 reasons, (1) no worry about missing your flite and (2) Islands slowing down, Athens always lively in fall.
I suggest Chania in West Crete, then Santorini. If your bargain Stanstead flite arrives EARLY in Ath A,P, fly via bargain Aegean Air to Chania. If LATE, take 9pm o'night ferry (u can get a budget berth in 4-berth all-F cabin).
Chania gorgeous-- interesting HIKES nearby, also beaches, good food, chance to interact w. locals. NOT winding-down since Crete is a year-round place. MUCH to explore for 5 days
Santorini: Bus to Heraklion Crete for the daily fast-ferry to Santorini for 3-4 days. Plenty of budget sleeps and eats to suggest in both places but not enough length allowed by Rick Steves! (PM me).
Athens - stay in Plaka district; athens is a totally safe comfortable big city for single F explore solo; great d-i-y solo walks too.
GET TRANSPORT FIRST -- Hotels/pensions afterward.