Updating referenced entities through entity_generate process plugin

FYI I've made adjustments to this patch as a custom plugin in my repo, and made it support entity translation updates, too. Thanks.

Here's my take on this, in a custom plugin:

<?php

declare(strict_types = 1);

namespace Drupal\sw_migrate\Plugin\migrate\process;

use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\Row;
use Drupal\migrate_plus\Plugin\migrate\process\EntityLookup;

/**
 * This plugin updates existing entities within the process plugin.
 *
 * All the configuration from the generate plugin applies here.
 *
 * Available configuration keys:
 * - langcode: (optional) If set, update translations instead of base entity.
 *
 * Example:
 * @code
 * destination:
 *   plugin: 'entity:node'
 * process:
 *   field_tags:
 *     plugin: entity_update
 *     source: '@_parent_id'
 *     entity_type: paragraphs_library_item
 *     langcode: '@_translation'
 *     value_key: id
 *     values:
 *       label: title
 * @endcode
 *
 * @see \Drupal\migrate_plus\Plugin\migrate\process\EntityLookup
 *
 * @MigrateProcessPlugin(
 *   id = "entity_update"
 * )
 */
class EntityUpdate extends EntityLookup {

  /**
   * The current row.
   *
   * @var \Drupal\migrate\Row|null
   */
  protected ?Row $row = NULL;

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $this->row = $row;

    $result = parent::transform($value, $migrate_executable, $row, $destination_property);

    return $this->updateEntity($result, $value);
  }

  /**
   * Update an entity for a given target.
   *
   * @param string $target_id
   *   The id of the entity.
   * @param mixed $value
   *   The entity values.
   *
   * @return int|string
   *   The entity id of the generated entity.
   */
  protected function updateEntity($target_id, $value) {
    /** @var \Drupal\Core\Entity\ContentEntityInterface $entity */
    $entity = $this->entityTypeManager
      ->getStorage($this->lookupEntityType)
      ->load($target_id);

    $entity_values = $this->configuration['values'];

    // Support for plugin-level translation config:
    $translation = FALSE;
    $langcode = $this->configuration['langcode'] ?? NULL;

    if ($langcode) {
      // If langcode is present in plugin config, expand it:
      $langcode = $this->row->get($langcode);

      // Unless value is false, get the language translation:
      if ($langcode !== FALSE) {
        $translation = TRUE;
      }
    }

    if ($translation) {
      foreach ($entity_values as $key => $value) {
        $value = $this->row->get($value);
        $entity->getTranslation($langcode)->set($key, $value);
      }
    }
    else {
      foreach ($entity_values as $key => $value) {
        $value = $this->row->get($value);
        $entity->set($key, $value);
      }
    }

    $entity->save();

    return $entity->id();
  }

}