The 20 minute sweet potato fries that came out raw in the middle

I followed a recipe that called for 400 degrees for 20 minutes, shaking halfway, and cut my sweet potatoes into what I figured were fry-sized sticks. The outsides got dark and crispy around the 16 minute mark but when I bit into one it was still hard and starchy in the center. Turns out my cuts were way thicker than the recipe meant, some pieces were nearly an inch wide, and sweet potatoes need more time than white potatoes to cook through. Next round I'm cutting them to a quarter inch and going lower and slower at 375 for about 25 minutes. Has anyone figured out a cut size and temp that gets these crisp without burning the tips?