I don't plan on making butter on a regular basis, but I do plan on one more batch once this one is gone. I will inoculate the same heavy cream with a liberal amount of commercial cultured buttermilk, let it set at room temperature for 1-2 days, gauged by the amount of sour smell, then do just as I did on this one. It will be interesting to see if my results with that one are more similar to yours with the yogurt culture. Since the bacterial content of yogurt and buttermilk is different, I suppose the resulting butter might also be.
Whatever, I'm having fun fooling with it. It has triggered many pleasant memories of cherished family members from my childhood, long since gone.
Spence