Sort buckets by shared field values

[Domme (Dominik Luebbers)]

I would like to group documents based on a group field G. I use the „field aggregation“ strategy described in the Elastic documention to sort the buckets by the maximal score of the contained documents (called 'field collapse example in the Elastic doc), like this:

{
  "query": {
    "match": {
      "body": "elections"
    }
  },
  "aggs": {
    "top_sites": {
      "terms": {
        "field": "domain",
        "order": {
          "top_hit": "desc"
        }
      },
      "aggs": {
        "top_tags_hits": {
          "top_hits": {}
        },
        "top_hit" : {
          "max": {
            "script": {
              "source": "_score"
            }
          }
        }
      }
    }
  }
}

This query also includes the top hits in each bucket.
If the maximal score is not unique for the buckets, I would like to specify a second order column. From the application context I know that inside a bucket all documents share the same value for a field F. Therefore, this field should be employed as the second order column.

How can I realize this in Elastic? Is there a way to make a field from the top hits subaggregation useable in the enclosing aggregation?

Any ideas? Many thanks!