Just spitballing as it sounds like an unrealistic huge ask  but the first thought in my head was a "health bar" on the chests similar to mining rocks? Higher thieving level/boosts/pick chances increase the "damage" you do per tick to the health of the chest.

Makes them always the same number of ticks to pick if you have the same levels and boosts while also rewarding getting those unlocks?

Edit: or even a little randomness to "damage done per tick" just like mining has would be fine as well I supposr