'Do Not Disturb' is all you need to save yourself from screen dystopia
Screen Time is a new iOS 12 feature that automatically keeps track of how much you use your iPhone every day. One main use-case here is to push you to use your device less per day, and actually experience your daily life as opposed to staring at your screen all day.
However, if you want to spend less time on your iPhone, there's a much easier and elegant way built into iOS.
Turn on 'Do Not Disturb', permanently.
This will prevent any notifications from waking up your phone screen and distracting you from whatever you were doing in the moment. You can still configure it to allow certain phone calls or messages through.
Aside from the annoying persistent alert on your lock screen informing you that Do Not Disturb is on, this is the most seamless way to spend less time on your phone and more time living.