Sounds delicious! I had never thought of using potatoes in focaccia but I’m sure it makes it very moist and tender. That btead can be a little dried out since it is thin and cooked until it is crispy on the edges.
I always watched my mother make it with potatoes growing up. From the region in Italy she is from, the focaccia is thick. It usually rises from 1 1/2 - 2 inches high. When I told her I started using leftover mashed potatoes, she thought it was a great idea.