Preheat the oven to 350°F (175°C). Place 4 cups of the lightly crushed Fruit Loops in a shallow dish. Line a cookie sheet and set it near the stove.
In a large bowl, whisk together 7 large eggs, 1 cup granulated sugar, 1 cup (4 oz) all-purpose flour, 1/4 cup whole milk, 1 teaspoon ground cinnamon, and 1/4 teaspoon kosher salt until the mixture is smooth. Pour the batter into a shallow baking dish suitable for dipping.
Preheat a large nonstick skillet over medium-high heat.
Working with 1 or 2 slices at a time, dip each 1-inch-thick brioche slice into the batter. Let each side soak for 5 to 10 seconds, then lift the slice with a slotted spatula and allow excess batter to drain back into the dish.
Place the dipped slice in the preheated skillet and cook about 3 minutes per side, or until the batter is set and both sides are golden brown. Transfer the cooked slice to the lined cookie sheet. Repeat dipping and cooking until all 10 slices are cooked.
After the slices are cooked, whisk the remaining 1/4 cup all-purpose flour into whatever batter remains in the baking dish to thicken it.
Generously brush the tops of the cooked French toast slices with the thickened batter. Invert each slice into the shallow dish of crushed cereal and press lightly so the cereal adheres. Brush the other side with more thickened batter and press that side into the cereal as well. Return the cereal-coated slices to the lined cookie sheet, pressing additional cereal into any sparse spots as needed.
Bake the cereal-coated French toast in the preheated 350°F oven for 5 to 7 minutes, or until the batter is fully set and the bottoms are lightly browned.
Serve warm, drizzled with maple syrup and sprinkled with powdered sugar.