Thank you, Carol now retired!
Exactly how strenuous is this tour? Moderate, intense?
I wouldn't say our Best of Istanbul tour was strenuous or intense. By my standards it was moderate; Even with the use of the tour-provided transit pass (and on one day, the tour bus) to cover larger distances from A to B, I still walked at least 6.5 miles each day and as much as 12.5 miles. That includes a walk I took each morning before breakfast and walking during free time. I'm not one to go back to the hotel to relax. There were some hilly streets, but mostly it was uneven walking on cobblestones or pavement. I was very glad I had sturdy shoes with good support.
One lady on our tour walked slowly with a cane and it was too much for her, so our guide helped her to arrange a driver (which she paid for) to take her to our sites.
Is Istanbul safe in 2023?
I felt safe there in 2022. Of course, there are incidents, as in nearly any big city.
Can credit cards be used?
Yes, for many things. But smaller vendors might not take them for (say) a keychain or a small street snack. So you will want some Turkish Lira. There is a lot of information about cash and ATMs in my original trip report:
The Best of Istanbul tour is a great way to see the city! Especially if it's your first time to Europe.