Magento 2 usage of UNION in a custom query?

I am working on a module to remove images of all deleted products, which is not the standard M2 behavior. My initial query to found images path still used by products was :

$connection = $this->connection->getConnection(); 

$select = "SELECT {$connection->getTableName('catalog_product_entity_media_gallery')}.value"   
    ." FROM {$connection->getTableName('catalog_product_entity_media_gallery_value_to_entity')}"  
    ." , {$connection->getTableName('catalog_product_entity_media_gallery')}"  
    ." WHERE {$connection->getTableName('catalog_product_entity_media_gallery_value_to_entity')}"    
    ." .value_id = {$connection->getTableName('catalog_product_entity_media_gallery')}.value_id"    
    ." UNION"  
    ." SELECT {$connection->getTableName('core_config_data')}.value"  
    ." FROM {$connection->getTableName('core_config_data')}"  
    ." WHERE {$connection->getTableName('core_config_data')}.path"  
    ." LIKE '%placeholder%'";  

return $connection->fetchCol($select);

I use PHP_CodeSniffer from magento-coding-standard and result is a warning for a potential raw SQL. To fix this, I finally use this query :

$connection = $this->connection->getConnection();  

$placeholders = $connection->select()
    ->from('core_config_data')  
    ->where('core_config_data.path LIKE "%placeholder%"')  
    ->reset(Zend_Db_Select::COLUMNS)  
    ->columns('value');  

$sqlImages = $connection->select()  
    ->from(['gallery' => 'catalog_product_entity_media_gallery'])  
    ->join(  
        ['to_entity' => 'catalog_product_entity_media_gallery_value_to_entity'],  
        'gallery.value_id = to_entity.value_id'  
    )  
    ->reset(Zend_Db_Select::COLUMNS)  
    ->columns('value');  

$select = $connection->select()->union([$sqlImages, $placeholders], Select::SQL_UNION_ALL); 

return $connection->fetchCol($select);

Same result but I wonder if it is not possible to compact it in a single query like the first one?
(all the examples i find in the M2 core are always combinations of several queries)