phplist doesn't provide a way to alter the order of sending emails.
You can look at domain throttling, see the config.php file, if you want to restrict the rate at which phplist sends to each domain, but in my experience it slows down the actual send rate significantly.
I understand that phpList doesn't include that feature. Could you guide me to the scripts that contain the sending logic? Some developer guide? I have some PHP progamming knowledge, so I will try to make the modification to shuffle the batch to deliver on each run.