Hi Lizzie, I can see you have a very challenging teaching situation. I’ve never taught in your context so I am wary of giving advice, also, most of the research on feedback is in more conventional contexts. However, there is little doubt that most feedback methods can be adapted for your students. I would try ‘spoof assessment’ or ‘anonymised assessment’ with your students, where you give students work done by ‘a student last year’ though you may have made it up. When students assess this there are not egos at stake. You can use this to develop ideas in your subjects, and to prepare students for a new art project by looking at what other students have done in response to similar projects.
Targets must be tailored to the student, so ‘staying in the lesson’ might well be appropriate.
You might find the following book useful: Westwood, P. “Common Sense Methods for Children with Special Educational Needs: Strategies for the Regular Classroom”

I agree with the thrust of your comment and thanks for it! I don’t think students need necessarily set formal targets for improvement as a part of snowballing necessarily. A lot of learning during snowball just comes from students learning informally from each other, they hear ideas they had not thought of themselves, and they have to think hard about the topic they are learning during discussion if the task is well designed and if teacher manages behaviour etc well.