Stir together paprika, garlic powder, cumin, chile powder, and 3 teaspoons of the salt in a small bowl. Rub mixture evenly all over brisket. Arrange onion slices in bottom of a slow cooker; place brisket, fat cap side up, on top. Cover and cook on LOW for 8 hours.
Remove brisket from slow cooker; discard onion, or reserve for tacos or another use. Let brisket stand until cool enough to handle, about 15 minutes. Cut brisket across the grain into about 1/4-inch-thick slices. Cover to keep warm.
Whisk together eggs, milk, pepper, and remaining 1 teaspoon salt in a medium bowl until well blended. Pour mixture into a large nonstick skillet; add butter pieces. Cook over medium-low, stirring often with a silicone spatula, until egg curds are set but remain tender, 5 to 7 minutes.
Sprinkle 2 tablespoons cheese onto each tortilla; top each with a scant 1/4 cup scrambled eggs. Divide brisket evenly among tacos; top evenly with pico de gallo (about 1 tablespoon each), and serve.