In a bowl, whisk together olive oil, lemon juice, minced garlic, oregano, paprika, salt, pepper, and red pepper flakes until well combined. Cut the chicken into bite-sized chunks and toss them in the marinade. Let marinate for at least 15 minutes or up to 45 minutes.

While the chicken marinates, prepare the salsa. Combine cucumbers, tomatoes, bell pepper, red onion, parsley, and feta in a mixing bowl. Season with lemon juice, salt, and pepper. Toss gently and let sit for about 10 minutes.

Heat a skillet over medium-high heat. Add marinated chicken in a single layer and cook for about 3-5 minutes until golden brown, then flip and cook for another 3-4 minutes until fully cooked with internal temperature at 165°F.

Warm the pitas over a direct gas flame or in a skillet for about 30 seconds per side, or wrap in foil and heat in the oven at 350°F for 5-10 minutes.

Assemble the tacos by filling each warmed pita with the cooked chicken, top with salsa and a dollop of tzatziki sauce. Serve immediately.