Line a large baking sheet with parchment paper or wax paper. Make sure the sheet is large enough to spread the snack mix into an even layer once it has been coated.

Add the popcorn, mini pretzels, roasted salted peanuts, candy corn, Reese's Pieces, and fall-colored M&M's to a very large bowl. Toss gently so the ingredients are distributed without breaking the popcorn.

Place the white chocolate baking chips, vanilla melting wafers, or almond bark in a microwave-safe bowl. Microwave in 20–30-second intervals, stirring thoroughly after each interval, until smooth; stir in the shortening or coconut oil if the coating seems too thick.

Drizzle half of the melted coating over the snack mixture and fold carefully with a spatula. Add the remaining coating and continue folding until most of the popcorn and other ingredients are lightly covered.

Spread the coated mixture onto the lined baking sheet in an even layer. Separate large clusters gently, but avoid crushing the popcorn or pressing the candy pieces into the coating.

Leave the tray at room temperature for 20–30 minutes, or until the white chocolate coating feels firm. Break the snack mix into bite-size clusters and transfer it to an airtight container.