I'm just wondering if skills ignoring 4s and just 25% chance of a failure per dice might not be a little too generous? With a skill of 1 you're going to have 74% chance of clean success, which raises the skill immediately, and then your chance of clean success is 94% with a skill of 2..!
And even if the DM is adding 2d4 every roll, you get 52% of clean success at skill level 1, and only 16% chance to make any cauldron dice back as the DM! At skill level 0 with 6d4, you still have the 52% chance of success, but only 18% chance of a clean success so there's that! But I'm not sure of the overall experience that those distributions are going to yield!
I think if I try it I'll see how it feels at first but my first design pass on this would be to really iterate on the ebb and flow of the cauldron and the DM moves tied to that, maybe really go full DM-moves list a la PbtA?