IMO inventory system shouldn't be +rows/+columns and instead work similarly to i.e tarkovs, where each new inventory expansion instead creates a different box of its own. So a bark backpack might grant you an extra 3x3, a leather one a 4x4, merchant robes might grant you two small 2x2's, etc.
Obviously all part of the same "inventory" ui to prevent a floating mess of windows.
Though if this is done, rotating items in inventory will be a mandatory addition


