Look at bendalloy, though. Let's say you spend a day drinking a hundred liters of water and filling up your bendalloymind.
At the end of the day you're still at normal hydration, even though you drank most of niagra falls. If you then go into a desert and get really thirsty, you can tap your bendalloymind and refresh yourself - and you'll stay refreshed even after you've stopped tapping.
It would be far more useful for Bronze to be storing 'mental energy' or 'sleep'. (IE: you sleep while storing bronze and end up getting less rested, but you can later tap for extra hours of sleep). As is, you're always going to end up with a worse ratio of time spent conscious to time spent unconscious. Which seems kinda unbalanced.
The whole 'you go into a trance, neither getting more tired nor less' is very different from how feruchemy normally works as well. The entire ability is bizarre.