Yep - Products Price Manager is precisely defined for giving quantity discounts to individual products.
If you want to discount bulk purchases for entire categories or all items, you'd use my plugin, the Quantity Discounts contribution.
(And, as you observed, to discount bulk purchases of a product with particular attributes, you'd use the Attributes Controller.)

There are a lot of options for quantity discounting in Zen Cart!

https://docs.zen-cart.com/user/produ...ity_discounts/