Adjust oven rack to lowest position and heat oven to 425°F (220°C). Spray rimmed baking sheet with cooking spray. Add oil to prepared sheet and tilt sheet until surface is evenly coated with oil. In a small bowl, combine salt, paprika, garlic powder, and cayenne; set aside.

In a large microwave-safe bowl, whisk water and cornstarch, making sure no lumps of cornstarch remain on bottom of bowl. Microwave, stirring every 20 seconds, until mixture begins to thicken, 1 to 3 minutes. Remove from microwave and continue stirring until mixture thickens to a thin yogurt-like consistency. (If necessary, after microwaving, add up to 3 tablespoons additional water to achieve correct consistency.)

Add rice flour and baking soda to cornstarch mixture and whisk to combine. Add cut sweet potato sticks and toss until each is evenly coated. Transfer fries to prepared baking sheet, arranging them in a single layer with one flat cut side down, and space between each piece. (Some cornstarch mixture will remain in bowl.)

Bake until bottoms are beginning to brown and fries easily release from pan, about 20 minutes. Remove sheet from oven and, using a thin spatula, loosen the fries and carefully flip taking care not to tear. Rotate pan and return to oven to continue to bake until second sides are golden brown, about 15 minutes.

For the Fry Sauce: While fries bake, in a small bowl, whisk together mayo, ketchup, sriracha, vinegar, and garlic powder.

Sprinkle fries with prepared salt mixture and toss fries to distribute seasoning evenly. Transfer fries to paper towel–lined platter and let cool about 5 minutes. Serve fries with prepared sauce, if desired.