Arrange racks in upper and lower thirds of oven; preheat to 350°. Line 2 large baking sheets with parchment.
In a medium bowl, whisk flour, baking soda, cinnamon, and salt.
In a large bowl, using an electric mixer on medium-high speed, beat butter, granulated sugar, and brown sugar until light and fluffy, 2 to 3 minutes. Add eggs and vanilla and beat to combine. With mixer on low speed, slowly mix in dry ingredients until just incorporated. Beat in M&M’s, peanuts, oats, and pretzels on low speed until just combined.
Scoop 16 (1/4-cup; 65-g.) dough and arrange on prepared sheets 3" apart. Slightly flatten balls with the palm of your hand.
Bake cookies, rotating sheets front to back and top to bottom halfway through, until golden brown and edges are set, 16 to 18 minutes. Let cool completely.