I'm making cumin-scented quinoa with shredded beets and the recipe calls for a sumac-yogurt topping plus sumac in the quinoa
sumac adds a tart note, so I'd sub with a little lemon juice and zest
SInce Sumac adds a bit of a lemony taste to many Middle eastern recipes, I would try a bit of Lemon zest.
If you have Zaatar, another Mideastern spice, it contains Sumac and would add a nice flavor
If you can get pomegranate molasses, that would be v nice here!