It's very difficult to alter forms of inline (content blocks) placed via Layout Builder

I tried using Block Class module and didn't work for me. My issue was that I couldn't use the same validate and submit functions that were working in the block edit page, but not when using layout builder. I ended up using something like this:

/**
 * Implements hook_entity_insert().
 */
function embeddocument_block_entity_insert(EntityInterface $entity) {
  _embeddocument_block_entity_insert_or_update($entity);
}

/**
 * Implements hook_entity_update().
 */
function embeddocument_block_entity_update(EntityInterface $entity) {
  _embeddocument_block_entity_insert_or_update($entity);
}

/**
 * Custom callback used by insert and update.
 * 
 * @param $entity
 * @throws \Drupal\Core\Entity\EntityStorageException
 */
function _embeddocument_block_entity_insert_or_update($entity) {
  if (\Drupal::moduleHandler()->moduleExists('block_content')) {
    $entity_langcode = $entity->get('langcode')->value;
    if ($entity->bundle() == 'document') {
      $entity_array = $entity->toArray();
      if (isset($entity_array['field_documents'])) {
        $mid = $entity_array['field_documents'][0]['target_id'];
        if (!empty($mid) && !empty($entity_langcode)) {
          $media = Media::load($mid);
          // If translation doesn't exist, create it.
          if (!$media->hasTranslation($entity_langcode)) {
            $media->addTranslation($entity_langcode, $media->toArray());
          }
          $media->save();
        }
      }
    }
  }
}