Top-hits on aggregations

Hello,
I need to get the top-hits over an aggregation.
In my use case I store SQL queries as documents (with different statistics like the time it tooks). Each querie is hashed, so that I can make an aggregation term on it.
I then have "groups" of exact same sql queries, on which I can compute aggregations, like the average time each query took.

Now I want to get the top-k queries w.r.t mean time it tooks. How can I do so ?
If i use a bucket_sort, I know that elastic will sort buckets after having extracting them, and because the number of bucket is limited, I do not have the guarantee to have top-k on this aggregation metric.

This is the code I came with so far:

GET /sql_requests_parsed/_search/
{
  "size": 0,
  "query": {
    "bool": {
      "must": [
        {
          "exists": {"field": "request"}
        }
      ]
    }
  },
  "aggs": {
    "same_requests": {
      "terms": {
        "field": "hashed_request",
        "size": 2000
      }, 
      "aggs": {
        "request": {
          "top_hits": {
            "size": 1
          }
        },
        "mean_call": {
          "avg": {
            "field": "callDuration"
          }
        },
        "sorting": {
          "bucket_sort": {
            "sort": [
              {"mean_call": {"order": "desc"}} 
            ]
          }
        }
      }
    }
  }
}

NB: I also tried "collapse", which seemed to fit my usecase at first glance, but I can't aggregate on the resulting inner hits.

I cannot find a good solution to this, if you know something I would be glad to hear it.