Help with migration needed - entity_lookup of multiple taxonomy references

I am trying to fetch nodes with multiple taxonomy references from Drupal 9 instance A to instance B. B already has the vocabulary terms, so i want to use entity_lookup in the process for this field to get the correct ids on B. The vocabularies on A and B have slightly different names (A: 'schulstufe' B: 'schulstufen')

My YAML:

item_selector: data/
source:
  fields:
    -
      name: school_levels
      label: 'School Levels'
      selector: /relationships/field_level

process:
  # does not work yet
  field_school_school_levels:
    plugin: sub_process
    source: school_levels
    process:
      target_id:
        plugin: entity_lookup
        entity_type: taxonomy_term
        bundle: schulstufen
        bundle_key: vid
        value_key: name

Source example from --migrate-debug output:

"school_levels" => array:2 [
  "data" => array:2 [
     0 => array:3 [
       "type" => "taxonomy_term--schulstufe"
       "id" => "b20f2930-b5bf-42c6-9a9a-4f701fbbf1ca"
       "meta" => array:1 [
         "drupal_internal__target_id" => 28
       ]
     ]
     1 => array:3 [
       "type" => "taxonomy_term--schulstufe"
       "id" => "404b35fb-ed2a-4f48-965f-7b88acab98d2"
       "meta" => array:1 [
         "drupal_internal__target_id" => 33
       ]
     ]
   ]

Destination example from --migrate-debug output:

"field_school_school_levels" => array:2 [
    "data" => []
    "links" => []
  ]

I went through all the explanations i could find, including https://git.drupalcode.org/project/migrate_plus/blob/HEAD/src/Plugin/migrate/process/EntityLookup.php but i have no clue what my mistake is. This must be a very common migration task, so i hope somebody here can help!