From your initial post, I would suggest that you consider sticking to your plan. We have an 8 day trip planned this spring with 5 nights in Athens and 4 nights in Santorini. We have been to Athens (our 6th trip) , Santorini (this will be our 3x) and Mykonos (2x). In 2023 We did Athens, Santorini, Naxos and Mykonos. 14 nights. We flew into Santorini, ferried to Naxos then to Mykonos and flew out of Mykonos. We have been to many Cycladic and Dodecanese islands.
We always fly into Athens and do our thing …Greek food, wine, archeology , and then fly back from an island the day before we fly home and overnight at the Sofitel. It’s literally a 10 minute walk from room to Departures. So, I don’t know anything about Hydra but a ferry to/from the Cyclades, to me, would be insane.
Santorini is fabulous. There are many ways to avoid crowds. Stay in Fira not Oia. Book a hotel room with a Caldera view. Take a wine tour. Take an Akratori tour. We find moderate priced restaurants with views…so meals aren’t expensive.
For Mykonos, we love Delos and stay right at the town square, a 15 minute walk to the old pier to boats Delos. We only stay in Delos 2 nites, one day. Everything is expensive, so spend up and enjoy it.