WP Query: Prioritize 'in_stock' Condition for Product Filtering

To achieve the desired result, you can modify the 'in_stock' condition in the elseif block as follows:

} elseif ($key == 'in_stock') {
    if ($value) {
        $meta_query[] = array(
            'relation' => 'AND', // Set the relation to AND
            array(
                'key' => 'qqp_qty',
                'value' => array('0'),
                'compare' => '>'
            ),
            array(
                'key' => 'qqpd_price',
                'compare' => 'EXISTS' // Only consider products with price
            )
        );
        $keys[] = 'in_stock';
    }
}

By adding an additional array to the $meta_query array, you can set the relation to AND and include the condition for qqpd_price to consider only products with a price.

Note that this assumes you have a qqpd_price custom field for products. If the field name is different, please adjust it accordingly.