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?
If you use a 9X13 pan, I'd just use 1 layer of apple slices. That's about all you'd get with the 2 apples called for in the recipe. They don't come out tasting sour, guess it's from the sugar in the topping and cake underneath.
It might take me a couple more tries to get the streusel the way I want it. Kind of like a crumble, a little crispy, a little soft in parts. It's got quite a bit of flour and sugar.
I was thinking I could cut in about 3/4 cup cold butter with my pastry blender, then lightly toss with just a little cold water to make some of it stick together but not as much as for a pie. I used to make quick crumble crusts for my pies for years, and that worked great.
For my crumble on pies (only it didn't have any sugar in the pie dough so that has to be factored in), I'd make a big 9X13 pie, blend the crust like big peas but wait to sprinkle the water.
Finish the remaining mixture for the pie cutting in the butter like it should be for pies (only I was using Crisco), toss with cold water, press out the shell and put in filling, then sprinkle the still dry crumble which doesn't have water added yet, spritz well with cold water (not too much, use a spray bottle), and the sugar (I use the coarse white sugar - baker's) I sprinkle on the top will stick and top brown prettier. It came out just a little crunchy; golden browned, everybody loved it.
Maybe check out some youtube videos. If the streusel has nice texture but a little skimpy, just double it. Oatmeal and honey helped in some muffins I make, but I prefer to perfect the streusel without the oatmeal for the apple cake.
Good luck and write back if I made it too confusing.
Too much butter and water might make it too heavy, so consider that. I've got myself thoroughly confused.
Making pies the way I devised and used for years was because I hated rolling crusts and wasn't good at it and the patching. And having a pin sock and pastry cloth to wash that got greasy. The dough you press in the pan is more forgiving and you can start pressing it in the pan right away without chilling it. First I would build up the sides nice and neatly, level off on the top, then sprinkle the rest in the bottom and press it down, trying to keep it even and taking care to blend well with the sides you have built. Rubbing with a baggie worked great for that but they don't make baggies any more. I finally found some sandwich bags that don't zip and will work for me (I only zip when I have to) and use a lot of the small bags. I found some at Walgreens finally but how long they will have them, who knows?
The dough has a little more loose flour than should be if you are going to roll your crust. Of course there should be no loose flour on the bottom crust after the water is sprinkled in that. A little loose flour or tiny crumble left in the top won't matter.
Sorry I got carried away. I loved the old Crisco for my pies. Now I hate it and I've been learning to roll better. But I do not want to use part butter. They taste blah so now I am going to try butter Crisco and may go back to hand pressing pies in the pan because it's not so stressful and messy.
My pressing method took a few tries to perfect; I could probably do it blindfolded now. But the other stuff.