20 days in greece

Crete is worth at least a week, possibly more depending on your interests. If you are into swimming and beaches, September/October is good as the water is still warm and weather can be less hot. April/May could be pleasant and the wildflowers would be out. If you want to visit the many archaeological sites in Crete I’d chose the spring.

I'm currently in Crete and it is hot, visiting the outdoor sites can be quite uncomfortable in the heat so it would be advisable to visit in the cooler months.

I'm staying in a villa with a pool on a relaxation holiday so the heat isn't an issue and the morning excursions are tempered by an afternoon around the pool or on a beach. If coming in the summer months I would absolutely be looking at spending the afternoons in and around water in which case more time will be required to explore the island. I'm here for two weeks so definitely taking a leisurely pace.

Spring or early fall are the best times to come to Greece. Fewer people, cheaper accommodation prices, and ideal weather.
These are always the times when I go to Greece.

In May the vegetation is in bloom, the weather is perfect and the days last longer. Only the sea is still a bit cool if you intend to swim.

The high season now lasts until around September 15/20. So if you want to avoid the crowds don't come before this date. Although Greek hospitality is excellent you may also find people tired after 3 months of dealing with tourists during the high season.

Since you have 20 days, you can easily stay at least 8 or 10 days in Crete. The island is very large and diverse.

As for the other islands, like all the people who come here for advice, you think of Naxos and Paros which are the best known (apart from Santorini and Mykonos which have turned into theme parks for cruise passengers).
But know that there are 20 other visitable islands in the Cyclades just as interesting and beautiful as
Naxos and Paros which, although different, are relatively similar.

Maybe you could replace one of these two islands with another like for example Amorgos

https://amorgos.gr/

Or an island of the small Cyclades like Schinoussa or Koufonissia

http://www.koufonisia.gr/?lang=en

Or an island in the Western Cyclades like Sifnos or Serifos

https://www.roomsinsifnos.com/

https://www.discover-serifos.com/en/

I have never met anyone who regretted having gone to these islands, even among those who already knew Naxos or Paros

The only "difficulty" is to organize your ferry trips because you don't go from one island to another without having done some research on how to get there.

Websites like Ferryhopper

https://www.ferryhopper.com/en/

Openseas

http://www.openseas.gr/

or GTP.gr

https://gtp.gr/RoutesForm.asp

allow you to find the right ferry at the right time.

In any case, assuming you fly home from Athens, save the visit to Athens for the end of your stay and stay there for at least 2 days.