Sort products without thumbnail in WooCommerce shop page

I would try to rename the custom function and add meta_query too! So it’d be like this:

function my_shop_custom_products_query( $q ) {
        $q->set( 'meta_key', '_thumbnail_id' );
        $q->set('orderby', 'meta_value_num');
        $q->set('order', 'DESC');
        $q->set( 'meta_query', array( array(
           'key' => '_thumbnail_id',
           'compare' => '>=',
           'value' => '0'       
        )));
}

add_action( 'woocommerce_product_query', 'my_shop_custom_products_query' );

If for some reason it didn’t work, then try to replace meta_value_num with meta_value and see if that’d work!