Preheat your oven to 350°F. Grease and flour two standard-sized loaf pans, or line them with parchment paper for easy removal. Set aside.
Mix the wet ingredients: In a large bowl, beat together 1 cup vegetable oil, 3 eggs, 1 3/4 cups granulated sugar, and 1 teaspoon vanilla extract until smooth and well combined. It should look thick and pale yellow.
Combine the dry ingredients: In a separate bowl, whisk together 3 cups all-purpose flour, 2 teaspoons cinnamon, 1/2 teaspoon allspice, 1 teaspoon baking soda, 1/2 teaspoon baking powder, and 1 teaspoon salt. Whisk thoroughly so the leaveners and spices are evenly distributed.
Make the batter: Pour the dry mixture into the wet mixture and stir with a spatula or wooden spoon until just combined. The batter will be thick — that's normal. Gently fold in 2 cups of squeezed, grated zucchini until it's evenly distributed. Don't overmix; a few streaks of flour are fine.
Prepare the cinnamon-sugar topping: In a small bowl, stir together 1/2 cup granulated sugar and 1 teaspoon cinnamon until uniform.
Layer the loaves: Divide half of the batter evenly between the two prepared pans. Spread it out, then sprinkle about half of the cinnamon-sugar mixture evenly over the batter (about 2 tablespoons per loaf). Spoon the remaining batter on top, spreading it to cover the cinnamon layer. Finish by sprinkling the remaining cinnamon-sugar mixture on top.
Bake to perfection: Place the pans in the preheated oven and bake at 350°F for 45–50 minutes. The bread is done when a knife or toothpick inserted into the center comes out clean, and the top is deep golden brown with a crackled sugar crust. If the top is browning too quickly after 40 minutes, tent loosely with foil.
Cool completely: Let the loaves cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely before slicing. Warm bread is delicate — wait the full time for clean, beautiful slices.