Santorini's steps and hills on a bad knee?
I was on Santorini 2 years ago. I have a collapsed flat foot and can appreciate your situation. Yes, there are stairs but there are also some graded paths. There is no way to avoid bad footing, pavement that has not been replaced in 2000 years, and crumbling stairs. Just take your time, watch where you are walking, and don't step backward. You might bring a folding cane just in case. Also, bring some euros for the public toilet. There is an attendant who has no mercy.
Santorini is just gorgeous and Oia is on the cover of every Greek tourism pamphlet. It is worth taking a reasonable risk. Facing the church in the square, going uphill to the right gives you the spectacular, if conventional, views of the island, sea, and church. Again faciing the church, going downhill to the left gives you a different perspective and there are many colorful buildings. There is an amusing Best Western hotel, tucked into an old building, it's easy to miss their small sign. Also, don't neglect walking down to the ramparts and looking out to sea. Equipment: at least 24mm FX WA and wider if you can. Consider occasional use of a polarizer, depending on direction you are pointing the camera; but at times a polarizer darkens the naturally brilliant blue sky too much so take shots both ways.
The St. George winery (not in Oia) was just terrific: quality for the price of tasting and the hospitality.