Preheat the oven to 450°F (230°C). Line a large baking sheet with foil or parchment if desired.
Arrange the cubed chicken, bell peppers, red onion, and mixed vegetables in a single layer on the prepared baking sheet.
In a small bowl, mix 4 cloves minced garlic, 2 tbsp olive oil, 2 tbsp fresh lemon juice, 1 tsp dried oregano, 1 tsp dried basil or thyme, 1 tsp kosher salt, and 1/2 tsp black pepper to make the marinade.
Brush or toss the chicken and vegetables with the marinade until evenly coated.
Bake for 15–20 minutes, until the vegetables are charred in spots and the chicken is cooked through and tender.
While the chicken bakes, make the tzatziki: combine 1 cup plain low-fat yogurt, 1/4 cup grated cucumber (with peel), 1 clove minced garlic, 2 tbsp lemon juice, 1 tbsp extra-virgin olive oil, 1 tbsp chopped fresh dill or basil, 1/2 tsp salt, and 1 tsp ground pepper in a small bowl; whisk until smooth.
Divide the cooked brown rice evenly among 4 meal-prep bowls (about 1/2 cup each if using 2 cups total).
Portion the roasted chicken and vegetables into the 4 bowls.
Cut the 2 whole wheat pitas into slices and place 4 slices into each bowl.
Portion tzatziki into 4 small containers (about 4 oz each) and store with the bowls in the refrigerator.
To reheat, remove pita, microwave the chicken and veggies until hot for 2–3 minutes, then serve with cold tzatziki.