To get all the details on the public bus routes on Santorini, here's the best resource I've found, including maps, photos, videos, and narrative:
Santorini Public Bus - Routes, Schedules, Tickets, Map (santorinidave.com)
And other Santorini info you might find of interest:
Santorini Greece - The 2024 Travel Guide (santorinidave.com)
Santorini, Rhodes and Mykonos are some of the easiest cruise ports to do it yourself and have an amazing day quite inexpensively, although it helps if you don't mind walking alleys and cobblestones since they are all built that way. They are three of my favorite ports ever, though. And I've been a lot of places.