Previous rules are good for some forms inserted by various plugins. For example I had a subscribe form and the action url is using http schema.
Additionally I added this gravatar rule too.
//gravatar
$buffer = preg_replace('|http(s)?://(.+).gravatar.com|i', 'https://$2.gravatar.com', $buffer);
Hello! I had the same problem and solved it with another plugin called “Force HTTPS srcset”
