I know this is an older thread, and you may have found your answer, but I accidentally noticed the quick solution ... it's NOT obvious.
Also, I have a newer pixel, but I assume that if your OS stays updated, this should work:
When you press your UP or DOWN buttons, for a moment you will see a dialog (it only stays there for 2-3 seconds, so you may have to press one of the volume keys again).
At the bottom of the dialog, you will see 3 dots. Press that ... ah HA!
You get separate Media, Call, Ring, Notification and Alarm volume controls.
That's the fast access. You can also get there more slowly through the SETTINGS->Sound & Vibration menus.