My 8-month-old male kitten, who had a habit of urinating on box-shaped objects, seems to have stopped after being neutered. However, he’s urinating on a lot of laundry due to bins resembling boxes. Spot treatment isn’t viable due to dried stains. Seeking a laundry detergent or additive to eliminate odor. I’m currently using Nature’s Miracle Urine Destroyer with Arm & Hammer OxiClean, but I’m unsure of its effectiveness. I need a long-term solution for all laundry loads.
Have you considered getting rid of the kitten 
This is one hella long-term/permanent solution
Use Vinegar Wash:
Mix one cup of white vinegar with one cup of water (1:1 ratio).
Coat the urine-stained area with this solution and let it sit for a few minutes.
Optionally, cover the stained area with baking soda (which absorbs and neutralizes odor).
Wash the urine-stained items in a washing machine set to cold or lukewarm water, without adding laundry detergent.
Air-dry the items afterward.