Great question!
Hmm, transformers might be a bit tricky here compared to queries. Queries have success handlers where you can trigger filtering on success, and they also have an .isFetching property that you can watch to see if the query is completed.
Are you using a transformer since JS queries don't have a way to trigger automatically? I wonder if we could use this workaround in order to use a JS query & then trigger the filtering action on success of the JS query. Let me know if you think this could work or if you have another reason to use a transformer.