I’ve searched the blog and found that both Harvest King and King Arthur Unbleached All Purpose have 11.7% protein content. Does that mean I can use these flours interchangeably and get the same result? If so, I’ll start using Harvest King because it costs way less than King Arthur.

It depends whether Harvest King consistently has a protein content of 11.7%. I have heard that some supermarket brand flours vary from season to season in protein content, but I don’t know whether Harvest King does. Try using it. If you like the results over time, stick with it.