Just visited yesterday! Important things to note when considering whether to cancel your tour to Oia. Yes as Biker has mentioned, check how many ships are in port. A normal day is 3-4 cruise ships. With that amount of passengers, queues will not be terrible, though you still have to queue. A bad day is above 5 cruise ships. Lines will be very long for the cable car.
We were the 2nd or 3rd tender off the ship and when we arrived at Fira (where the cable car starts from), the queue was already snaking. We waited about half an hour to 40 minutes to get to the front. My tip is to try to be on the first tender if you do want to continue to see Oia. Oia is lovely and beautiful, Oia is also where to best see the 3 blue domed churches, but it will be crowded with narrow walkways at times. We docked when there were only 3 ships tendered that day and already crowds were an inconvenience at times.
My advice is if there are fewer ships in port the day you are at Santorini then go for the experience. The views are breathtaking and it is easily now my family's favorite port in the Mediterranean.
With mobility issues, I saw a young teenage girl brought by her father in a wheelchair all the way up the mountain. So if she can do it in a wheelchair, I'm sure it's possible for you too. Just maybe travel less and see Oia on your own instead of with a tour group.