Combine brownie mix, vegetable oil, water, and eggs in a large bowl. Mix with a wooden spoon until well blended. Spread into the prepared baking pan. Bake in the preheated oven until a toothpick inserted 2 inches from the side of the pan comes out clean, 28 to 30 minutes. Allow to cool, then cut into bite-sized pieces.
Meanwhile, whisk together milk, 1 cup peanut butter, and pudding mix in a large bowl until smooth and thickened. Set aside.
Beat cream cheese, powdered sugar, and vanilla extract in the bowl of a stand mixer fitted with the whisk attachment until smooth. Gradually add heavy cream, scraping down the sides of the bowl as needed. Beat until stiff peaks form.
Fold cream cheese mixture into the pudding mixture until combined.
Place remaining ½ cup peanut butter in a small microwave-safe bowl and heat in the microwave for 30 seconds or until melted; set aside.
Layer 2 cups brownie pieces, about 1 ⅓ cups chopped peanut butter cups, and ¼ cup peanuts in the bottom of a 4-quart trifle dish. Drizzle with about 2 tablespoons melted peanut butter. Spread about 3 cups peanut butter filling over the top. Repeat the layers two more times.
Top with the remaining brownie pieces, peanut butter cups, and peanuts. Cover and refrigerate for at least 2 hours or up to overnight before serving.
