I'm going to suggest a solution to my own problem here, which is to modify the Create Document node

// Convert specified keys in data to document references based on docRefs object
    if (docRefs && typeof docRefs === 'object') {
        Object.keys(docRefs).forEach(refKey => {
            const refValue = data[refKey];
            if (refValue !== null && typeof refValue === 'string' && refValue !== '' && refValue !== 'null') {
                // Use the value in docRefs to determine the collection for the document reference
                data[refKey] = firestore.doc(

);
            } else {
              // if the docref was passed but is empty, remove it to avoid an invalid ref
              delete data[refKey];
            }  
        });
    }