Combine cucumber, shallot, vinegar, sugar, crushed red pepper flakes, 1/4. tsp of the salt, and 1/8 tsp. of the black pepper in a bowl; gently toss. Set aside.
Heat a large nonstick or cast iron skillet over medium-high.
Drizzle olive oil evenly over flesh of salmon. Season with sumac, lemon zest, cumin, remaining 1/4 tsp. salt, and 1/8 tsp. black pepper. Place salmon in pan, skin-down, and cook 8 minutes, until skin is crispy and fish is nearly cooked through. Flip, cook 2 minutes, and remove pan from heat.
Prepare yogurt spread by combining all ingredients in a small bowl; stir with a whisk.
Warm tortillas according to package directions. Break salmon into chunks and divide evenly onto each of 8 tortillas. Top evenly with cucumber mixture and coriander-mint yogurt. Garnish with fresh herbs, if desired.