YOu should check more than the price on Sky Express, and you will see that it is Not a bargain. They are small planes, and you are only allowed one small bag to carry on, plus a handbag or laptop -- and by "small bag" I do not mean a 20" rollaboard -- I mean something like a gym bag, that would weigh no more than about 15 lb. If you have a standard 20 or 22" suitcase it must be checked and can cost you $50. Read the small print.
If y ou are flying on the right day, you might get an Aegean "Promotional price" to Santorini if you are lucky. Go to the website https://en.aegeanair.com/plan/book-a-flight/ and tick the box +/1 7 days. You'll see a week's worth of flights. The lower prices ae usually 7 am or sometimes 9 pm or later however, so if you want midday you must pay the piper. And no, it's not too early to book a flight IF you'll arrive on a popular day for working folks -- Sat. or Sunday... because many will want those midday flights. If you book the "semi-flex" fare, you'd be able to change to another flight without penalty.