I had an idea for something that could work in the game pretty easily code wise.
When talking to the equipment storage NPC in the spire, instead of a whole new GUI list and array for storage, he could instead enable a flag for (a) hidden extra character(s) named equipment storage, the dummy character would appear in your characters list and be useless for battle at 0/0 hp with a universal restriction on item usage. The icon of the extra character(s) could be just plain text "Storage".
Since Sandra has the ability to wield multiple daggers, and that implies the developer has the ability to designate slots, equipment slots for the storage characters could just be tailored for as many equipment slots as GUI limitations allow.
Before anyone asks, yes I know you have inventories for weapons and armors, but the whole point is to save some of the more useful items from raids that aren't legendary from being dismantled, which will happen if they are not equipped.
Edit: I found out that when I collect the monstergirl material box in the Aldlyn castle, though it displays 16 salamander tails being collected, it actually increases the amount in my inventory by 100 tails. Perhaps this amount was used in development but accidentally left in?