Other than the timing of actually brewing potions, I've figured out a way to use the cauldron that somewhat works for me. I agree it would be best to be able to use the potions when I want during the week, not just when I remember to activate the cauldron before playing for the day, which I feel gives me the most benefit since it's all random.
I use up to half of the points I earn each week to add ingredients and brew potions - mostly just randomly pick ingredients and brew when the success rate goes to 50% or greater. Most brews are successful on my two higher level characters.
Then I use the remaining half of the points I earn each week to enhance the levels. I never use resources or diamonds (sorry Inno) to level them up.
Over time this has allowed most of the potions that get brewed on my Traders of Unar level and Embassies characters to have a decent amount of time on them so they are almost useful, and also a decent boost percentage for whatever they are good for. They have about 4,000ish points each week to divide up between potions and enhancements so most enhancement levels are getting in the level 20-25 range now. I do the same for Tarea, my beta character in Fairies, but consider it more a building for the future kind of thing since her points each week are so limited, though she does get one or two potions a week.
One thing I do like is you don't have to enhance just the benefits from the potions you brewed - you can select any of the types available to your character. I love that you can use all the points, and any partial enhancement level stays until you can finish it off the next week. This way I can keep them more balanced.
I agree with the posters, that if they'd change the timing of the cauldron (which makes ZERO sense to me) OR allow you store and use the potions within a certain amount of days then the Cauldron would be received better.