@artvesta,
Backup your site before anything.
Replace the file /image-store/_in/galleries.php with galleries.php.
It will allow you to break your galleries into subfolders, you can name them whatever you want except ‘_resized’;
gallery/1
gallery/2
gallery/23
Once you break gallery into folders use the “scan folder” option to update the gallery information. You may have to increase you memory limit in the wp-config.php file define('WP_MAX_MEMORY_LIMIT', 250 );