Prepare the syrup by combining 12 ounces frozen mixed berries, 1½ cups granulated sugar, and 1¼ cups water in a medium saucepan.
Bring the berry syrup mixture to a simmer over medium heat. Simmer for 20 minutes; the sugar should have dissolved, and the berries should be bursting and breaking down.
While the simple syrup simmers, wash the lemons, then slice them in half, and use a citrus press to squeeze the juice out of the lemons into a liquid measuring cup to get 1 cup lemon juice.
Remove the syrup from the heat and allow to cool in the pot for 20 minutes, then strain through a fine-mesh sieve, squeezing the berries to extract as much juice as possible.
In a pitcher, combine the lemon juice, 1⅓ cups berry simple syrup, and 6 cups cold water.
Slice 2 lemons into 8 slices and add 4 to the lemonade, then stir to combine.
Serve in a highball glass with ice and garnish each with a lemon slice and berries. Serve with a straw.