Blueberry Coffee Cake - Testing

I started experimenting with sourdough. OMG so much discard, so much waste! I know sourdough discard gives good biology to the compost pile but I've been conditioned to not waste good food. So, when in doubt, make a dessert.
1 - No matter the results, this will remain in the testing phase for this first try. What I haven't worked out with the sourdough is timing. I decided to make this dish, then thought I ought to feed the sourdough, too, then thought oh but when do I want the stuff to be ready for baking and then that through me off. So I decided to do the optional sourdough-free version. But what I'm most curious about is the sourdough version!
2 - I did it with sourdough and it worked! But I added another variable so the results still need testing. Last time, I used a 9x9 glass baking dish and the cake took forever to cook, far more than the half hour in the instructions. The resulting cake was light, crumbly and a bit dry, perfect for eating with a warm beverage like coffee. This time I used an 8x11 ceramic gratin dish, thinking that a more spread out cake would be more mouth-sized and easier to pick up and eat with your fingers. I got the correct height of cake, but it came out moist and soft. I mean, oh, horror of horrors, I made a perfectly moist cake with sourdough! However, for the effect I'm seeking is different. I've made the required changes to the recipe for the next attempt.

The Cake
¼ cup butter
¾ cup sugar
1 egg
1-¼ cups flour
1 tsp baking soda
½ tsp cinnamon
½ tsp salt
½ cup milk
½ cups sourdough discard
1 1/2 cups blueberries, fresh or frozen
The Topping
¼ cup butter
¼ cup sugar
¼ cup brown sugar
⅓ cup flour
½ tsp cinnamon

1. Preheat oven to 350ºF. Grease an 8×8 9×9 inch or a 7×11 pan.
2. For the cake: Cream together butter, sugar and egg. 
3. Add flour, baking powder, cinnamon and salt. 
4. As you are mixing in the dry ingredients, pour in the milk and sourdough. (SEE NOTE) Blend well. 
5. Pour about 2/3 of the mixture into the prepared pan. Over this, evenly distribute 1 cup of blueberries. Add the remaining batter and the remaining blueberries. 
6. For the crisp topping, mix ingredients together well and sprinkle over batter in the pan.
7. 
   
8. Bake for 30-35 minutes or until lightly browned.

To make without sourdough:
Increase milk to 3/4 cup, flour to 1 1/2 cups and add 2 teaspoons baking powder. Omit sourdough. Combine as directed above.

(If you don’t have a stand mixer, add the dry and wet ingredients alternately.)