User may edit their webform submissions, but only while in draft
You don't need a custom route, you can hook into the entity access check for the existing entity routes:

mymodule.module:

use Drupal\Core\Session\AccountInterface;
use Drupal\Core\Access\AccessResult;
use Drupal\webform\WebformSubmissionInterface;

/**
 * Implements hook_ENTITY_TYPE_access().
 */
function mymodule_webform_submission_access(WebformSubmissionInterface $webform_submission, $op, AccountInterface $account) {
  if ($op == 'edit' &amp;&amp; !$webform_submission-&gt;isDraft() &amp;&amp; in_array('role', $account-&gt;getRoles())) {
    return AccessResult::forbidden()-&gt;cachePerPermissions();
  }
  return AccessResult::neutral()-&gt;cachePerPermissions();
}