I've found the best way to get a human at a service center is to be as friendly as possible to your SC rep, and keep up the relationship with them via email. At the two SC nearest to me, I've gotten a point of contact at both that I can email as needed. For one of them, I even have their personal cell phone number they told me to reach them because otherwise it is impossible to get a human at the SC otherwise.
Roughly, getting a human at a SC is an "off the menu" option, like the $35k Model 3.