Add a
html tag to WooCommerce checkout radio buttons form field options

Solution:

For custom checkout radio buttons fields, you can use woocommerce_form_field_radio composite hook to add a <br /> tag like:

add_action( 'woocommerce_form_field_radio', 'customize_form_field_radio', 20, 4 );
function customize_form_field_radio( $field, $key, $args, $value ) {
    if ( ! empty( $args['options'] ) && 'radio_choice' === $key && is_checkout() ) {
        $field = str_replace( '</label><input ', '</label><br /><input ', $field );
        $field = str_replace( '<label ', '<label style="display:inline;margin-left:8px;" ', $field );
    }
    return $field;
}

Code goes in functions.php file of the active child theme (or active theme). Tested and works.

Don’t forget to remove all <br /> from your options array.

So the display will be: