Unfortunately, Shortcuts currently only let’s you work with files, not folders in what might be termed a fully functional way.
I’ve had a quick look at utilising Scriptable to do the listing, but Dropbox is one of the many storage providers that doesn’t seem to support the functionality Scriptable needs to use - presumably file bookmark compatibility.
You could look at triggering some external script to generate such a list for Dropbox. Maybe on a remote desktop machine (via SSH). Or maybe a call to a web service like Zapier or Integromat could generate a sub folder listing as a JSON return to a webhook call?