I mean, my first reply addressed literally exactly what you said in the part I quoted.
Each item has a category. Assign each category to a specific retainer. Group categories in some way that makes sense to you. For example, I find the following groupings to work well:The problem is not knowing which stacks he already has.
- metal, stone, lumber
- cloth, leather, bone
- food, ingredient, reagent, fish
- materia, demimateria, catalyst, dye
Then it'll be easier to remember what's on each retainer.