Using water filter on a Cauldron filled with water should basically give a purified water for leather flask to refill. But heating it first with campfire or fire block.

Also when Leather Flask has like minimum use like 1 use or 2 uses. Just putting the liquid aside with the cauldron can be time saving.

The cauldron isn't that type of block in the game. It would require reworking significant minecraft game mechanics to even achieve this. I'm not going to implement this.