Adding a custom Shipping methods field to an order

Below is what I used in the end to solve my problem:

add_action('woocommerce_init', 'shipping_instance_form_fields_filters');

function shipping_instance_form_fields_filters()
{
    $shipping_methods = WC()->shipping->get_shipping_methods();
    foreach($shipping_methods as $shipping_method) {
        add_filter('woocommerce_shipping_instance_form_fields_' . $shipping_method->id, 'shipping_instance_form_add_extra_fields');
    }
}

function shipping_instance_form_add_extra_fields($settings)
{
    $settings['route_number'] = [
        'title' => 'Route Number',
        'type' => 'text', 
        'placeholder' => '1,2,3...',
        'description' => 'Enter route number'
    ];

    return $settings;
} 

add_action( 'woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta' );

function my_custom_checkout_field_update_order_meta( $order_id ) {
         $chosen_methods = WC()->session->get( 'chosen_shipping_methods' );
    $chosen_shipping = substr( $chosen_methods[0],-1);
         $free_shipping = get_option( 'woocommerce_free_shipping_'.$chosen_shipping.'_settings' ); 
        $route_number    = $free_shipping['route_number'];

        update_post_meta( $order_id, 'Route Number', sanitize_text_field( $route_number ) );

}