Preheat the oven to 275°F. Line two 13 × 18-inch rimmed baking sheets with parchment paper.
Combine the Peanut Butter Chex, Wheat Chex, pretzels, cheddar crackers, and Honey Nut Cheerios in a very large bowl.
Whisk together the melted butter, brown sugar, maple syrup, kosher salt, and cinnamon. Pour over the dry mixture and stir gently to coat.
Divide between the prepared baking sheets. Bake for 25–30 minutes, stirring every 10 minutes, until lightly golden.
Cool completely for approximately 30 minutes, or until dry and no longer warm.
Stir in the mini marshmallows, candy corn, and Reese’s Pieces. Store in an airtight container at room temperature for up to one week.