There isn't really much of a reason for the retainers to need to be summoned individually for storage. They should just remove the storage feature from the individual retainers and add an option on the bell to entrust/ withdraw items that has a number of tabs dependent on the number of retainers you have. That way you don't have to constantly check which retainer has which item when you go to deposit or withdraw.

They should also add the ability to search for items by name instead of only being able to do so by clicking on the item and hitting search. Sometimes I know i need a specific item from my retainers but I don't know which one has it and i cant just type in the name to be told who does.