Power down when screen not working

On Pixel phones at least, holding down the power button will force it to power off without requiring any screen interaction. You have to hold it down for a fairly long time (something like 30 seconds).

If you've set up adb debugging, I believe you can also run the shell command reboot -p (not sure whether that requires root though). I.e., connect it to a paired computer, and run adb shell reboot -p from the computer.