You don't get more bank slots when you buy an expansion. A lot of those materials can be stored in material storage which has a very generous amount of space, just press the button in the top right of your inventory screen to deposit to material storage with the leather/ingredients/etc in your inventory.
Yes I feel like most of the value in the expanders is lost because you only want to hoard maybe 10% of the items you are able to store in there. I would much rather use guild storage or even mules. Maybe buy 1-2 but don’t go crazy just so you can store 30k mithril like the guy above me stated. KTPhUC.