See also this thread
Our code mentioned there uses the collective.jsonify output with a custom importer (which is not based on transmogrifier). It contains custom migrators for FormFolder and Topics. The topic migration code is very well tested for the University Ghent migration where we process about 200 collections (Plone 4.3 to 5.2). The ZIP archive from the thread above is not completely up-to-date. Ping me if you need the most current version.