How do I create Comma Separated list of attached image ids?

You can try this one-liner, within the loop, instead of all your above code:

$ids = join( ',', wp_list_pluck( get_attached_media('image' ), 'ID' ) );

where we pluck out the ID’s from the get_attached_media() output.

Another approach would be to use:

$ids = join( ',', get_attached_media( '_image_ids' ) );

where we’ve introduced a new input parameter _image_ids, supported by the use of the following plugin:

/** 
 * Plugin Name: Enhance the get_attached_media() function.
 * Description: Support for the custom '_image_ids' parameter.
 * Plugin URI:  http://wordpress.stackexchange.com/a/169152/26350
 * Author:      birgire
 * Version:     0.0.1
 */
add_filter( 'get_attached_media_args', function( $args, $type, $post ) {
    if( '_image_ids' === $type )
    {
        $args['post_mime_type'] = 'image';
        $args['fields']         = 'ids';
    }
    return $args;
}, 10, 3 );