How long is the ship in Santorini for? If you are in for a long time, get off the ship in the afternoon!
I was also just on Reflection and the ship was in port until 10pm. With 5 ships in port I thought there is no way I am getting off in the morning. So we got off at 1pm - no queue at cable car at all! (but once we got to the top there was a huge queue waiting to go back down at this time.) We took a bus to Oia, and spent some time there. It was about 7pm when we got back to Fira for the cable car back down and again no queue.
So well worth it if you can avoid peak times!