1 or 2 more Cyclades Islands?

Hi from the west coast.
While tourism is very hot this season things tend to slow down in early October in the smaller islands. Therefore your plan to spend the last three nights in Athens is a good decision.
Crete is an excellent choice as it has so much to offer. Venetian era old towns cities, great beaches, and many ancient sites. You could spend a month in Crete and not see it all. Frankly I would suggest you use your additional 7 or 8 nights in Crete. We pretty much choose to visit West Crete with Chania our starting base. There are lots of sights and activities in this area and a rental car will allow you to explore more thoroughly. If you start your trip in Chania you probably would want to rent you car after spending the first few days enjoying The old town and harbour. No point in paying for a car when it will just be parked for the first few days.
Be sure to look for a locally owned car rental company. These companies work hard for your business and the service is outstanding . We have used AutoRentalCrets several times with great experience. We also used Athenscarcrete. They tend to have older vehicles with lower prices. Other recommended would be The Best car rental, Autoclub and Anna Cars. Its a good idea to compare what each of them offers.

The ferry from Crete to Santorini pretty much all depart from Heraklion so you will want to end up there at least the night before you ferry hop. There are several very good ancient sites in Cental Crete. Knossos and the museum are highlights. In the interior you can visit Phastos which is like Knossos but without the restoration. Nearby the site at Gortyn is a don't miss opportunity. There is a gated ancient site there but when you finish that site, wander down the road through the olive groves. There is a whole Roman city buried in those groves. Its a wonderful experience.

If you choose not to stay in Crete longer and do Santorini and are still looking for a week on another island. Naxos or Paros are both excellent choices. They are on direct ferry routes with several sailing daily. We always choose Naxos. Its our favourite island.

Here are some images
Chania 2022. https://www.flickr.com/photos/stanbr54/albums/72177720299646015

Western Crete https://www.flickr.com/photos/stanbr54/albums/72157632115757899
Gortyz https://www.flickr.com/photos/stanbr54/albums/72157666456665788

Around Naxos town 2022 https://www.flickr.com/photos/stanbr54/albums/72177720299959098
Back to Naxos 2022 https://www.flickr.com/photos/stanbr54/albums/72177720299940917

Before choosing one or two islands after Santorini you must take into account the ferry routes.

Santorini→Naxos→Paros is a route that you can easily find with several daily ferries with Seajets or Blue Star Ferries, even at the end of the season.

On the other hand, Sifnos is part of the Western Cyclades (with Sikinos, Folegandros, Milos, Kimolos Serifos, Kythnos) and is on another ferry route.

To put it simply, this means that from Santorini you will have to choose, either you go to toward Sifnos and the Western Cyclades or toward Naxos-Paros which are part of the Central Cyclades.

You didn't mention Amorgos, and I think you should add it to your list. It's 1h30 from Santorini with a daily Seajets ferry.
From Amorgos you can easily reach Naxos, which makes another couple of islands possible after Santorini

As for the Andros-Tinos couple, if it is easily accessible from Athens, after September 15 you will not have a ferry from Santorini that will take you there.

Thank you / merçi beaucoup Stan and JoLui! Your insight and suggestions are excellent and much appreciated.

If you are seeking a relaxed and comprehensive exploration of the Greek islands while maintaining a professional and efficient travel itinerary, I would suggest refraining from adding additional islands to your itinerary. Instead, I recommend extending your stays in both Crete and Santorini, allowing for a more immersive experience. It is advisable to minimize excessive transfers, as they consume valuable time, whether by ferry or plane. Opting for a minimum of two weeks in Crete, with accommodations strategically chosen in both the western region renowned for its stunning beaches and the eastern region boasting exceptional tourist attractions, would offer a well-rounded exploration. Additionally, dedicating approximately one week to Santorini is highly recommended, given the captivating sunsets, breathtaking panoramas, and the myriad of intriguing sights and activities available. By following this approach, you can ensure a balanced travel experience while maximizing your time and enjoying a seamless journey through the Greek islands.

Questo è un ottimo consiglio. Grazie mille a voi!