Had the same question as you regarding driving spice of za'atar. Hyssop in biblical scholar reference seems to be thought as origanum syriacum, implying a different herb from what I think of as hyssop in a botanical sense. All recipes I have seen cite thyme as the most commonly used herb with varying personal touches as described by other responders to your question.. Regardless, I love the different expressions of this seasoning and think of my friend who gifted me his Lebanese aunt's personal mix made from spices she gathered in Lebanon. .. Beautiful.