Filter Document Reference by its subcollection?

I have a collection with a subcollection of documents. When a user chooses some of the documents, they get added as a document reference to a field in the user document.

I'd like to filter by collection ("trail"), and count the number of documents ("chapters") added. But I can't quite seem to get the filtering to work.

I think I want to do a Backend Query of the Subcollection Documents, filtered by the Parent Collection. But I might be thinking about it wrong. Any suggestions are very welcome!