Hierarchical facet block, child-only selection

Here's the code I'm using so far:

namespace Drupal\mysite\EventSubscriber;

use Drupal\search_api_solr\Event\PreQueryEvent;
use Drupal\search_api_solr\Event\SearchApiSolrEvents;
use Drupal\taxonomy\Entity\Term;
use Solarium\Component\QueryInterface as SolariumQueryInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Event before a Search API query gets finally converted into a solarium query.
 */
class SolrEventSubscriber implements EventSubscriberInterface {

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events = [
      SearchApiSolrEvents::PRE_QUERY => 'preQuery',
    ];
    return $events;
  }

  /**
   * Modify the query before it's executed.
   */
  public function preQuery(PreQueryEvent $event) {
    $search_query = $event->getSearchApiQuery();

    // Only look at the Teachings page.
    if ($search_query->getSearchId() != 'views_page:search_teachings__page') {
      return;
    }

    $solarium_query = $event->getSolariumQuery();
    // Change the query to display both SOLR query and AI-boosted results.
    if ($solarium_query instanceof SolariumQueryInterface) {
      $filter_queries = $solarium_query->getFilterQueries();
      if ($filter_queries) {
        $changed = FALSE;
        $prefix = 'itm_node_topics:';
        /** @var \Solarium\QueryType\Select\Query\FilterQuery $filter_query */
        foreach ($filter_queries as &$filter_query) {
          $options = $filter_query->getOptions();
          if (empty($options['query'])) {
            continue;
          }
          if (str_contains($options['query'], $prefix) === FALSE) {
            continue;
          }

          $topics = [];
          foreach (explode(' ', $options['query']) as $key => $topic) {
            preg_match('/(.*)\"([0-9]*)\"/', $topic, $matches);
            if (!empty($matches[2])) {
              $topics[$matches[2]] = $matches[2];
            }
          }
          // No topics were found in this query, so try the next one.
          if (empty($topics)) {
            break;
          }

          // Loop through all of the topics, if one of them has a parent ID that
          // is in the list of topics then remove that parent item from the
          // topics list.
          $terms = Term::loadMultiple($topics);
          /** @var \Drupal\taxonomy\Entity\Term $term */
          foreach ($terms as $term) {
            $parents = $term->parent->referencedEntities() ?? NULL;
            if (empty($parents)) {
              continue;
            }
            if (in_array($parents[0]->id(), $topics)) {
              unset($topics[$parents[0]->id()]);
              $changed = TRUE;
            }
          }

          // If the topics list was changed, i.e. items were removed, then
          // rebuild the option variable.
          if ($changed) {
            $new_option = [];
            foreach ($topics as $topic) {
              $new_option[] = $prefix . '"' . $topic . '"';
            }
            $options['query'] = '(' . implode(' ', $new_option) . ')';
            $filter_query->setOptions($options);
          }
        }

        // If the arguments were changed, update the query.
        if ($changed) {
          $solarium_query->setFilterQueries($filter_queries);
        }
      }
    }
  }

}

I thought this would work - the results seem to be correct, but the facet now only shows the child and its parent & grandparent terms.

I tried to build a process plugin to accomplish this but wasn't able to work out the correct approach.

In the meantime I built the following, which technically works but might not be the best way:

namespace Drupal\mymodule\EventSubscriber;

use Drupal\facets\Event\FacetsEvents;
use Drupal\facets\Event\UrlCreated;
use Drupal\taxonomy\Entity\Term;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Modify Facets.
 */
class FacetsEventSubscriber implements EventSubscriberInterface {

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events = [
      FacetsEvents::URL_CREATED => 'urlCreated',
    ];
    return $events;
  }

  /**
   * Filter out parent terms on Topics facets.
   *
   * @param \Drupal\facets\Event\UrlCreated $event
   *   The event object.
   */
  public function urlCreated(UrlCreated $event): void {
    $facet = $event->getFacet();

    // Only work on the Topics facet.
    if ($facet->id() != 'topics') {
      return;
    }

    $url = $event->getUrl();
    $facetResult = $event->getFacetResult();

    $query_string = $url->getOption('query');
    if (!empty($query_string['f'])) {
      $new_query = [];
      $topics = [];
      $prefix = 'topics';
      foreach ($query_string['f'] as $key => $value) {
        [$facet_name, $term_id] = explode(':', $value);
        if ($facet_name != $prefix) {
          $new_query[] = $value;
        }
        else {
          $topics[$term_id] = $term_id;
        }
      }

      // Only bother with additional logic if there's more than one term.
      if (!empty($topics) && count($topics) > 1) {
        // Loop through the topics, if one of them has a parent ID that is in
        // the list of topics then remove that parent item from the topics list.
        $terms = Term::loadMultiple($topics);

        /** @var \Drupal\taxonomy\Entity\Term $term */
        foreach ($terms as $term) {
          $parents = $term->parent->referencedEntities() ?? NULL;

          // No parents, so just add the facet to the list.
          if (empty($parents)) {
            $new_query[$term->id()] = $prefix . ':' . $term->id();
            continue;
          }

          // If the parent is in the list then remove it from the list - this is
          // the whole purpose for all this logic.
          if (in_array($parents[0]->id(), $topics)) {
            unset($new_query[$parents[0]->id()]);
          }

          // Add the item to the list.
          $new_query[$term->id()] = $prefix . ':' . $term->id();
        }

        // Rebuild the option variable, after sorting it.
        sort($new_query);
        $query_string['f'] = $new_query;
        $url->setOption('query', $query_string);
        $event->setUrl($url);
      }
    }
  }

}