Hello @Ben_Page,
You only need to specify the field name in the Expand fields property if the column's type is Lookup, Person or Group, or Managed Metadata.
If a column type is Text, you only need to enter its internal name in the Extra fields property.
If you want the Description to be displayed directly in the dropdown list of the lookup field, you need to change the appearance of the search results. Please find more information in the documentation:
If you want the data to be displayed on the form after the user selects an option, you need to populate another field or control on the form. Please learn more here: