In a mixing bowl, whisk together the marinade ingredients: olive oil, lemon juice, minced garlic, oregano, paprika, salt, pepper, and red pepper flakes. Cut the chicken into chunks and coat them in the marinade. Allow to marinate for at least 15 minutes, or up to 45 minutes for deeper flavor.

While the chicken marinates, in another bowl, combine the salsa ingredients: diced cucumber, tomato, bell pepper, red onion, parsley, and feta cheese. Add lemon juice, salt, and pepper. Mix well and let sit for 10 minutes.

Heat a skillet over medium-high heat for about 2 minutes. Add the marinated chicken chunks without crowding. Sear for 3 to 5 minutes without moving, then flip and cook for an additional 3 to 4 minutes until browned and fully cooked.

Warm the pitas in the skillet alongside the chicken for about 1 minute per side or in the oven at 350°F for about 5 minutes.

Remove pitas from heat, assemble tacos with cooked chicken, salsa, and drizzle with tzatziki sauce. Serve immediately.