Hi,
Then create mass actions via the menu System>Mass actions and configure it with a filter on the product_manufacturer_id column equal to the id of a brand and an action to add a category corresponding to that brand and then another one to remove the categories corresponding to the other brands. Then set the trigger to "after a product is updated" and "after a product is create" and when you edit/create a product, a category corresponding to the brand will be added automatically to the product.
Or you could do that manually by seting a second category for the brands in your products if they don't change much.
Then, you can use the category restriction of your discounts.
And for the categories that you had to create for that, you can have them all in an unpublished category so that they don't appear on your frontend.