When I make tsatziki, I use at least 3 cloves of garlic and only yoghurt -- a thick kind, like Balkan style. Mind you, I know actual Greek people who swear by part yoghurt, part sour cream, but I prefer all yoghurt, tangier, and they like mine too. I suspect Japanese people aren't that used to garlic so they go easy.
The cumin in the meatballs sounds nice, gotta try that.