Just got back from Santorini on the Explorer. Tenders started around 1:30. We had tender ticket 7 which was the first non excursion boat (yours should be earlier with the Key). Our number was called around 2:30. Since we were the only cruise ship getting in at that time, the cable car up was empty. We hopped the local bus to Oia and were in line to take pictures with the blue domes by 3:40 and actually took the pictures 4:00. The ships that arrived in the morning had already been through Oia so we were just competing with tours from Explorer (who arrived to the domes after us ). One thing to note on your plan, the boats from the old port to Oia are an option but you will still have to climb up a hill to get into Oia (port is at sea level and Oia is about half the distance up the crater compared to Fira but still a hike).