Greece in mid-November

I wouldn’t go to Greece as late as mid November, as the chance of bad weather is too great for me to risk. I have been mid October and it was wet and blustery. Average weather for Santorini in November shows it gets about the same rain as London. Of course, you may get lucky...

Rainy season.

Santorini is very nice even at winter. Only Fira are open this period but as the view is so nice you would not mind the closed souvenir shops.
But November in Mykonos I would not go. There is almost anything to do.
I would advise you to visit instead Athens or a village like Parnassos Arachiva

If you want to avoid the hordes of tourists and high prices then going to Santorini/Mykonos may be good for November . . . however it won't be beach weather.