The advancing thaumaturge will produce a considerable number of wand foci, which would rapidly fill their inventory. To make the foci managable, they can craft a Focus Pouch, which can be carried anywhere in their main inventory, hotbar, or belt slot. The pouch is crafted on the Arcane Workbench, at moderate cost.
If used as an item (placed in the hotbar, wielded and right-clicked), the focus pouch will show a customized inventory which can only hold wand foci. Foci can be dragged in or out, or rearranged as desired.
When a player wishes to choose a new focus for their wand or staff (pressing F by default), they get a GUI overlay showing all available foci, including those within any carried focus pouch(es). Any focus removed from a wand or staff will automatically be placed in a focus pouch if one is available.