My trip plan started much like yours, except with Hydra in the mix On a ten day first Greece trip. I landed on Fira (3), Naxos (4), Athens (3), cutting Nafplion and Hydra from the mix. both will be on the next itinerary.
I am struggling now to plan three nights in Santorini. It might need four. Between lounging, catching sunsets, eating/drinking, land excursions (Akritiri, villages, wineries), a sea excursion (geology formations and points of interest) Oia (which is falling to the bottom of my list) , and Amundi Bay seafood, I’m out of time with three nights in Fira I’m flying to Santorini and I arrive at 1330 so I really only have 2.5 days. If you’re ferrying from Athens, it will be half of your first day.
I don’t know anything about Kastraki, but it seems the piece to cut out. I did just watch The RS Peloponnese video yesterday with Nafplion, Olympia, Mycaenae, and other places I cannot spell and it looks like a week or ten day trip on its own. If not cutting, Kastraki, extend your time there and save Nafplion for another trip. Then you could add to Santorini and probably also pickup Hydra. Some would argue for more time on Naxos. Greece is a lot bigger than I ever knew!