Place the slow cooker insert on a digital scale and zero it out. Add exactly 48 oz of raw chicken thighs. Pour the soy sauce, dark soy sauce, honey, mirin, ginger paste, and garlic paste directly over the meat. Toss until the chicken is fully coated.
Cover the slow cooker with the lid. Cook on HIGH for 4 to 5 hours (or LOW for 5+ hours). Do not open the lid during cooking to trap the steam and ensure the collagen fully denatures.
While the chicken cooks, rinse the 2.5 cups of dry short-grain rice until the water runs clear. Cook fully in a rice cooker or on the stovetop. Set aside.
Once the chicken is fully cooked and tender, remove the slow cooker lid. In a small bowl, aggressively whisk the 3 Tbsp of cornstarch with the 4 Tbsp of cold water until it forms a smooth, stark white liquid.
Pour the cold slurry directly into the hot, thin liquid inside the slow cooker. Stir gently. Leave the slow cooker uncovered for 15 to 20 minutes to allow the residual heat to activate the starch, thickening the liquid into a glossy glaze.
Using metal tongs, shred the tender chicken thighs directly inside the thickened teriyaki sauce. Add the fully cooked rice on top of the shredded chicken.
Sprinkle the chopped green onions and sesame seeds over the rice. Fold the mixture aggressively until the white rice completely absorbs the dark glaze and the chicken is evenly distributed.
In a separate bowl, whisk together the low-fat mayo, 0% Greek yogurt, sriracha, salt, pepper, and garlic powder. Add milk 1 tablespoon at a time, whisking continuously, until the sauce cascades smoothly.
Divide the chicken and rice matrix evenly across 8 meal prep containers. Drizzle the Yum Yum sauce in a tight zigzag pattern over each portion. Secure the lids for storage.