I usually add freshly chopped mint leaves, salt, pepper, sweetener and a little bit of white wine vinegar (all to taste) for a syn-free mint yogurt dressing.
Even easier would be mixing some syn-free mint sauce into your yogurt and then add salt, pepper and sweetener to taste. But I am not sure if there are any syn-free ready made mint sauces out there. I'll check for you, I'm intrigued now x
Some time ago I posted a recipe for tzatziki on here. That might go well with your kebab, too?