We were on the same cruise. With regards to Santorini, yes there were 4 ships in port, which is really not ok.
I don't know who makes the decision on how many ships are allowed in daily, but it ruins the experience somewhat, especially if you are unprepared.
We had a private tour picking us up at the top of the cable car at 8am, so my husband was 2nd in line for tender tickets early in the morning, which meant we were on the 1st tender and the 2nd cable car. Our tour then went directly to Oia, which we enjoyed immensely with very small crowds, on our way out of Oia 30 minutes later the crowd was shoulder to shoulder flowing in.
We then spent the day driving around the island with many uncrowded stops, a wine tasting and a delightful lunch in a waterside local restaurant, chosen by our driver.
Our return to ship was via the steps. We arrived back in Fria at around 2.30, even though all aboard was 4.30, as we knew there would be a line for the cable car, however we never did see where the end of that line was, it snaked away around the laneways, so we decided to take the steps. Tempers were flaring with people in the line trying to stop us walking through to the steps as they thought we were jumping the queue!
I really enjoyed the walk down the steps, we were not in any hurry, so we took it very slowly and carefully, but I also heard someone had been hurt and our departure was delayed as their belongings were packed, we saw suitcases being offloaded onto a small boat shortly before we finally sailed away.
You can plan well for getting up the top early, or on a ship tour, but it seems everyone is dropped at the top to navigate their own way down. So if it is a busy day, plan for a long wait in the cable car line or a slow careful walk down the stairs (my pick).
It also helped that of the 4 ships in port that day, we were the first there, so we beat the crowds going up.