Combine the dairy base. Add the sour cream, Greek yogurt, and mayonnaise to a medium bowl. Stir together until smooth and fully combined.
Add the garlic and liquids. Press the garlic directly into the bowl. Add the Fanta, pineapple juice, pepperoncini brine, and ketchup. Pour the soda slowly to manage the foaming reaction with the dairy.
Stir gently but thoroughly. Mix until the liquids are fully incorporated and the sauce has turned a uniform pale pink or soft coral color.
Add the dry ingredients and relish. Spoon in the sweet pickle relish, then add the paprika, kebab spice, cumin, oregano, black pepper, salt, MSG and cayenne if using.
Mix well. Stir until all spices are evenly distributed throughout the sauce. Taste — it should feel balanced between creamy, tangy, gently sweet, and warmly spiced.
Adjust seasoning. Add a little more salt, a touch more brine for acidity, or a pinch more paprika for depth as needed.
Rest the sauce. Cover the bowl with plastic wrap and refrigerate for a minimum of 30 minutes. Overnight (8–12 hours) is strongly recommended and produces a noticeably better result.
Stir before serving. Give the sauce a good stir before using. Serve cold or at room temperature. Store in a sealed jar or airtight container.