They just need to get rid of the slot layout for the retainer... Turn the retainer into an organized list structure with unlimited slots.

Inventory management in this game really is absurd. I've had to resort to discarding or selling off every that is not level 50, and then when I need to make something that is lower level I go to the board, buy the mats I need, make the item and discard any remaining mats... just to keep my inventory somewhat managable...