// move out of stock products to end of loop in WooCommerce Archives

add_action( 'woocommerce_product_query', 'custom_woocommerce_product_query' );
function custom_woocommerce_product_query( $q ) {
    // Get any existing meta query from the query
    $meta_query = $q->get('meta_query');

    // Add our condition
    $stock_status_meta_query = array(
        'relation' => 'OR',
        array(
            'key' => '_stock_status',
            'value' => 'instock',
            'compare' => '=',
        ),
        array(
            'key' => '_stock_status',
            'compare' => 'NOT EXISTS', // This ensures compatibility with variations
        ),
    );

    // If there's already a meta_query, we add ours to it. If not, we create a new one.
    if (!empty($meta_query)) {
        $meta_query[] = $stock_status_meta_query;
    } else {
        $meta_query = array($stock_status_meta_query);
    }

    // Set the modified meta query back
    $q->set('meta_query', $meta_query);

    // Order by stock status first
    $q->set('orderby', array('meta_value' => 'ASC', 'date' => 'DESC'));

    // This ensures out-of-stock products are listed last
    $q->set('meta_key', '_stock_status');
    $q->set('order', 'ASC');
}