Athens deserves at least 2 days with three better.
Still with just 8 days I would drop one island and save Athens for the end of your trip.
It will be very difficult and time consuming to combine Hydra with either Santorini or Mykonos as there is no direct ferry from Hydra to either island. You'll have to go back to Piraeus and then take a ferry to one of the other islands and hope there is a ferry available without wasting a lot of time waiting for that ferry.
There are numerous other Greek Islands that offer what you are looking for with less tourists, lower prices and maybe a bit more "Greek" than Santorini/Mykonos.
More research on your part is needed to see what other Greek Islands have that may work for you.