Add the flour, cinnamon, baking powder, baking soda, and salt to a large bowl. Thoroughly mix these dry ingredients.
In a separate large bowl, whisk together the 0-fat Greek yogurt, milk, and vanilla extract until the yogurt is smooth and there are no lumps. Then, whisk in the eggs and sugar or sugar-free sweetener. *Add up to 1/4 cup of milk as needed for consistency, or up to 1/2 cup if using a sugar substitute. The batter should be thick but spreadable, not stiff.
Add the wet ingredients to the bowl with the dry ingredients a little at a time until fully combined. Make sure not to overmix the batter.
To make the streusel crumb topping, mix the flour, brown sugar, cinnamon, and salt. Then, incorporate the light butter and use your hands or a fork to combine everything and break up the butter. Do this until the crumb topping comes together and has a crumbly consistency.
Lightly spray a nonstick loaf pan with oil or you can use parchment paper. We used a 9x5 loaf pan. Pour half of the batter into the loaf pan. Sprinkle a thin layer of the crumb mixture over the top of the batter, and save the rest for the top of the cake. Then, pour the rest of the batter on top. Top the cake with the remaining streusel crumb topping.
Bake the coffee cake at 350℉ for 37-40 minutes. You will know it is done when a toothpick comes out clean from the center. Let the coffee cake slightly cool, carefully remove it from the pan, slice it into 8 pieces, and enjoy!