Hey @MattH,
Unfortunately dgraph only generates the aggregate<Field> queries for lists of Types or Interfaces (https://dgraph.io/docs/v21.03/graphql/queries/aggregate/#advanced-aggregate-queries-at-root).
However you can easily achieve what you need with Lambda resolvers. Have a look at this document: https://dgraph.io/docs/v21.03/graphql/lambda/query/
Let me know if you need help.