Automatically load pending draft
There is. When a user's saves draft a token gets generated. You need to get their token value.

$webform_submission = \Drupal\webform\Entity\WebformSubmission\WebformSubmission::load($sid);
$token = $webform_submission-&gt;getToken();


Then, you need to generate a URL like this

$draft_url = '/form/webform-name?token=' . $token


and on login redirect them to that url.