mongodb pipeline $filter with condition not $in

I want to use a pipeline to update a document by filtering on the keys of sub-documents in an object.

eg data:

{
  things: {
    a1: {},
    ...
    x1: {}
  }
}

Where "things" can contain variable keys/properties. I know I can use an update pipeline like this, where "keepKeys" are the keys I want to keep in the "things" object:

[ 
  $set: {
    things: { 
      $arrayToObject: {
        $filter: {
          "input": {
            $objectToArray: "$$ROOT.things"
          },
          "as": "item",
          "cond": {
             "$in": [
               "$$item.k",
               keepKeys
             ]
          }
        }
     }
  } 
]

This works, but I'd rather filter out the things I don't want as this is a much shorter list, eg where discardKeys are the keys I don't want:

[ 
  $set: {
    things: { 
      $arrayToObject: {
        $filter: {
          "input": {
            $objectToArray: "$$ROOT.things"
          },
          "as": "item",
          "cond": {
             "$nin": [
               "$$item.k",
               discardKeys
             ]
          }
        }
     }
  } 
]

But $nin is not supported in pipeline $filter cond

This works, as per @Wernfrie Domscheit

[ 
  $set: {
    things: { 
      $arrayToObject: {
        $filter: {
          "input": {
            $objectToArray: "$$ROOT.things"
          },
          "as": "item",
          "cond": {
            "$not": {
              "$in": [
                "$$item.k",
                discardKeys
              ]
            }
          }
        }
     }
  } 
]