How is Greece (Athens, Mkyonos, and Santorini) from midoctober to late?
You may get some sunny days, but I have also had wall to wall rain and gale force winds for days. It wouldn’t be my choice for late October.
At the end of October there will be a little less choice as some hotels and restaurants close (but Mykonos and Santorini have visitors all year round)
The problem could be the weather which can still be excellent or there can be very strong winds in Aegean Sea which stop all ferry traffic.
(I was on Amorgos in autumn 2019, there were no ferries for 3 days)
So, it's very random.
I travel to Greece every October, usually the first half, but have gone once the second half.
For the most part the weather is still very nice, mild temperature, sunny skies and a bit of rain.
However, there is no guarantee what the weather will be like.
You'll be going after the height of the High Season, but Santorini/Mykonos still gets lots of tourists and they'll still be a good selection of shops, markets and tavenas.
It doesn't matter when you go to Athens. It's a city of millions so you'll have more than enough options for services.
I went to Greece in October 2018. Among the 3 locations you mention, I was only in Athens. One day in Athens there was some light to moderate rain. There was an astonishing rainbow in the distance in Delphi and I thought I heard faint distant thunder. Another day in Greece I might have seen brief very light rain. Most of the time the weather was marvelous. On one or more evenings when I was outside I might have felt the temperature get cold enough to need a extra long-sleeved shirt or light jacket. You can search this site for weather data in various places in the last ten years:
Early October 2021, weather was beautiful. But the winds were strong and steady for 2 weeks from Santorini all the way over to Monemvasia. They even have a name for it, Meltemi.