Line two large baking sheets with parchment paper or silicone mats. In a very large mixing bowl, combine the popped popcorn, pretzels, nuts, and cereal (if using). Gently toss together and set aside.

In a medium heavy-bottomed saucepan, combine the butter, brown sugar, corn syrup or maple syrup, and fine sea salt. Cook over medium heat, stirring gently, until the butter melts and the mixture is smooth and combined. Bring to a gentle bubble and cook for 3–4 minutes, stirring occasionally, until thickened and glossy but not dark or burnt.

Remove the pan from the heat and quickly stir in the vanilla extract, ground cinnamon, and nutmeg (if using). The mixture may bubble up slightly.

Pour the warm caramel in a slow, steady stream over the popcorn mixture, tossing gently with a long spatula or wooden spoon as you go. Continue tossing from the bottom of the bowl up to coat as many pieces as possible. Work fairly quickly, as the caramel will start to set; small clusters are fine.

Divide the caramel-coated mixture between the two prepared baking sheets and spread into an even layer. After 1–2 minutes, while still warm but not hot, sprinkle the apple chips and caramel bits evenly over the top and gently press them into the mix so they stick. Let cool completely at room temperature for 20–30 minutes until crisp.

Melt the white chocolate chips or melting wafers in a small microwave-safe bowl in 20–30 second bursts, stirring between each, until smooth. Drizzle the melted white chocolate over the cooled snack mix in thin lines. If desired, finish with a light sprinkle of extra cinnamon or flaky sea salt. Let the chocolate set completely, 10–15 minutes, then break the mix into large chunks and transfer to a serving bowl or airtight container.

Store the cooled caramel apple snack mix in an airtight container at room temperature for up to 5–7 days. For longer storage, it can be frozen for up to 1 month; thaw at room temperature in a closed container.