Also I thought maybe I would make it in my big tube pan if the center keeps sinking. It almost fills a 9-inch springform pan. Otherwise I like the look of it just the way it is.
I’m kind of thinking maybe I should try your cake and apple part with a different streusel recipe because the way you prefer your topping sounds like what I like. Also, a list of cake pan volumes might come in handy—like maybe exchanging the springform pan for a 9x13 and just changing the baking time might resolve the sinking-in-the-middle issue?