Loops through your custom module and ensures the config on hook_rebuild. Features like behavior, sans-Features.

mymodule.php

<?php

/**
 * Implements hook_rebuild().
 */
function mymodule_core_rebuild() {
  $collection = \Drupal\Core\Config\StorageInterface::DEFAULT_COLLECTION;
  $config_manager = \Drupal::service('config.manager');
  $modules = ['custom_module1', 'custom_module2', 'custom_module3'];
  foreach ($modules as $module) {
  
    $default_install_path = drupal_get_path('module', $module) . '/' . \Drupal\Core\Config\InstallStorage::CONFIG_INSTALL_DIRECTORY;
    $storage = new \Drupal\Core\Config\FileStorage($default_install_path, $collection);
    $prefix = '';

    $data = $storage->readMultiple($storage->listAll($prefix));
    if (!empty($data)) {
      $dependency_manager = new \Drupal\Core\Config\Entity\ConfigDependencyManager();
      $config_names = $dependency_manager
        ->setData($data)
        ->sortAll();

      $overrider = $config_manager->getConfigCollectionInfo()->getOverrideService($collection);
      foreach ($config_names as $name) {
        if ($overrider) {
          $new_config = $overrider->createConfigObject($name, $collection);
        }
        else {
          $new_config = new Drupal\Core\Config\Config(
            $name,
            $storage,
            \Drupal::service('event_dispatcher'),
            \Drupal::service('config.typed'));
        }
        if ($data[$name] !== FALSE) {
          $new_config->setData($data[$name]);
          $new_config->set('_core.default_config_hash', \Drupal\Component\Utility\Crypt::hashBase64(serialize($data[$name])));
        }

        if ($entity_type = $config_manager->getEntityTypeIdByName($name)) {
          $entity_storage = \Drupal::entityTypeManager()
            ->getStorage($entity_type);

          if ($storage->exists($name)) {
            $id = $entity_storage->getIDFromConfigName($name, $entity_storage->getEntityType()->getConfigPrefix());
            $entity = $entity_storage->load($id);
            $entity = $entity_storage->updateFromStorageRecord($entity, $new_config->get());
          }
          else {
            $entity = $entity_storage->createFromStorageRecord($new_config->get());
          }
          if ($entity->isInstallable()) {
            $entity->trustData()->save();
          }
        }
      }
    }
  }
}