Since common V-belts for cars and such are so cheap, I'd slap a new belt on there before throwing the parts away, and here's why............
See how the outer side of the belt is nestled down inside of both pulleys, as opposed to being even with the outer circumference of both of the pulleys? I'd bet dollars to donuts that it's because the belt's sides have been worn away to the point of changing a 4L belt to a 3L, just like I've seen literally hundreds of times in my career in HVACR. The reason you can't stop it from slipping under load is highly likely to be due to the belt resting on the bottom of at the very least the rear, if not both front and rear pulleys.
They're called V-belts because the V-shaped configuration is supposed to mate to the V-shaped groove of the sheaves. The V-belt MUST ride on the sides of the belt, and NOT on the bottom of the belt. Once the belt is on the bottom of a pulley, you can never tighten it enough not to slip under heavy load, at least not without destroying your bearings.
You still should get a CVT in the end, but with a belt you can play with the thing until you do.