the beasts are very much like children, and we all know this. they never had proper childhoods, friends, or even real names besides those that described their duties (fount of knowledge, bringer of happiness, etc.). this is why they act impulsively and don't understand their emotions most of the time. so I really hate it when people act like the best way for ALL of them to get better is by force, aggressiveness, or lashing out, because their child-like mentality guarantees that doing that would only make things worse, especially for shadow milk or burning spice.
I know not many people agree with the idea of redemption, and you don't have to, but here is some ways I personally think they could be helped, even if slightly (for shadow milk I'll be going off his base self, not ultimate shadow milk because I have no idea how he would be helped atp besides figuring it out on his own).
mystic flour: debating respectfully with her and acknowledging her beliefs while discussing her revelations from a different perspective.
burning spice: perhaps showing him other ways to take out his wish for destroying things, along with showing or debating with him on how maybe not everything is temporary or useless.
eternal sugar: it would be hard to convince her of things considering she is good at twisting words, but she needs to be shown how some cookies don't gain happiness in the way that she does. she just needs to be taught that happiness is sweeter when there is passion and also pain to go beside it, even if not much of it, and that her way of happiness should be presented as a choice (she can keep her garden, but maybe restrain the powers that allow her to trap and trick cookies into staying).
silent salt: he needs to see that without bad, there is no good, and that nobody can be perfect or live up to the high standards that he considers good. he also needs to be taught more about sympathy, and taking other's reasons for acting the way they do seriously, instead of immediately choosing to be so judgemental just because he deals with his trauma different than others. I honestly believe that if convinced by someone he already trusts, he would be willing to be taught these things (someone like salt cellar).
shadow milk: he needs to be paid attention to often, and there's nothing wrong with that. also, he needs to get help with his trust issues and have cookies to believe/encourage his knowledge while also learning to think more critically about decisions he makes. I feel like he wouldn't appreciate having answers told to him directly, but rather should have it feel as if he is figuring things out on his own, since he usually believes what he thinks to be true more than anyone(which makes sense considering he is the former fount of knowledge). this goes for learning to trust people again too, he needs to be accepted and be able to slowly connect with people like pure vanilla.
PS: yeah, I know this is kinda dumb. feel free to disagree or comment/leave asks about what could maybe work better, but regardless, I did work really hard on this. hope you enjoyed <3