mix
糅 centers on blending things together into a single mixture. The core idea is combining ingredients or elements so they become one.
糅 combines 米 (rice) with 柔 (soft, gentle), suggesting the idea of mixing rice or grains until they become soft and blended. The exact historical development is uncertain, but the phono-semantic structure is plausible.
Rice 米 is mixed with something soft 柔 until everything is blended together. Picture stirring soft, cooked rice into a mixture.
For ジュウ, imagine mixing a thick stew: stew -> ジュウ, and the rice and soft ingredients blend into one.