Twelve liters of dry produce for one week? That's 1.7 liters per day! You might wanna rethink how much rice or oatmeal that would result in and cut back a bit - like by at least 50 to 75%!

Different people have different systems depending on if their panniers have pockets or are just one big bag. A stuff sack isn't an approach I use myself. Ziplocks are good for individual ingrediants. Those can be stored in pannier pockets, or inside the cooking gear itself, or inside a couple rectangular Tupperware containers. Tupperware gives a solid base to pack other things on top of, a container to use to mix things in, and something to temporarily store meats, salads or other special care items that need to be in spillproof containers and might need ice.

Any or all of those will work. Your cooking and organizational preferences should decide what works for you.