We did the same cruise on the 2nd-9th September.
Everything was amazing, ship, food, staff, shows, cabins etc.
The itinerary was just right BUT there were just so many people around the island. When we arrived in Santorini the were 4 other huge cruise ships and just couldn't enjoy somee of place due to the crowds. Never again will we go in high season. Everything says midv September sound be good, so fingers crossed.
I don't know how many in your party but here are some suggestions.
Mykonos, unless your are young and want party only, case hire or scooter hire a good option. Not a pretty island but Mykonos town pretty but expensive
Milos very quiet, the Crystal is the only ship that goes there. We took a bus around to a famous beach, can't remember the name. Very good
We did all the free excursions which were good. The only thing we did on our own was go to the volcano in Santorini just 20 euros fob the port of Feta.
Toy have to best in mind that the crystal is an old ship and not fancy line Disney, Princess lines etc
The one thing missing, in my opinion, is they do no give your any information on the ports you visit, no maps, tips etc. You are just left to your own devices so do your homework before you arrive
Have a lovely time