Can I use store-bought za'atar instead of making my own?
Yes — with one check. Most commercial blends have good dried thyme and sumac but use pre-ground sesame that has already lost its toasted aroma. If the za'atar smells dusty rather than herbal, add an extra teaspoon of fresh sumac and toast a tablespoon of whole sesame seeds separately, then stir both in. A good Palestinian or Lebanese brand from a specialist Middle Eastern grocer is genuinely usable; a supermarket generic is the one to doctor.