Smarty Upgrade cause fatal errors

The templates crash when I try to upgrade the Smarty version.

In the admin->system status I have the message:

Smarty Update (v2 => v4)

CiviCRM is updating a major library (Smarty) to improve performance and security and php 8.3 compatibility. The update is currently optional, but you should try it now.

To apply the update, add this statement to civicrm.settings.php:

  define('CIVICRM_SMARTY_AUTOLOAD_PATH',
    '/opt/drupal/vendor/civicrm/civicrm-packages/smarty4/vendor/autoload.php');
Some extensions may not work yet with Smarty v4. If you encounter problems, then simply remove the statement.

If I do that then try to find or create a new booking I get an error mesage. This is the new booking error:

 Sorry, due to an error, we are unable to fulfill your request at the moment. You may want to contact your administrator or service provider with more details about what action you were performing when this occurred.
Syntax error in template "file:/opt/drupal/web/sites/default/files/civicrm/ext/booking-main/templates/CRM/Booking/tpl/select-resource/add-source.modal.tpl" on line 1 "{crmScope extensionKey=""}{crmScope extensionKey=uk.co.compucorp.civicrm.booking}" - Unexpected ".", expected one of: "}"
Return to home page.

If I "comment out" line 1 and line 108 in https://lab.civicrm.org/extensions/booking/-/blob/main/templates/CRM/Booking/tpl/select-resource/add-source.modal.tpl#L1

by using {litteral}{/literal} tags (something I googled! https://www.smarty.net/docsv2/en/language.function.literal)

then i can move onto the next error.

I'm not sure if that's a fix or not??

If it is a fix, then I'm happy to go through the templates and do the work. Thanks