I’ve actually found my own answer, which is always nice 
I started by using a TEST inside the list (after insert last as), but this would leave me with empty list items because they would still be created.
By using a IF before the list, I can still filter the JSON, but then only add list items IF they meet the aforementioned condition, which in this case is a specific stage. But could be used to match any property.
I can then store that data in a var and populate the single list view when needed, without having to do lots of API calls 
