I'm taking some inspiration from the Beepberry with an always-on, or primarily-on, Pi Pico that is in charge of power-up and shutdown of the Raspberry Pi 3A+.  And I'm making some stuff up as it makes sense for this particular adventure: 

• Power Control and conservation:
  • The power button on the back is the hard-on and hard-off button.
  • The pico will run at battery voltage and the pi has the lipo shim to boost to 5v.  This gives independent control of the pico and pi.
  • When I push the power button, the pico will get power.  And I can push it to make sure stuff is off if the system hangs.  This is a complaint I have with the pimeroni on off shim - it won't turn the system off if it is hung and the OS doesn't toggle a gpio pin at the end of shut-down.
  • The pico will boot and then signal the Lipo shim on the Pi to turn it on.
  • The pico can signal shutdown to the pi and power it off.
  • The pico can toggle the "off" pin of the power switch to completely power down the computer.
• Activity Monitoring:
  • All of the user input aside from the touch screen is through the pi pico, so it can keep an activity timer.
  • The pico can signal to dim the display or turn off it's backlight after configured periods of inactivity.
  • The pico can signal to flicker the backlight if the battery gets low.
  • The pico can use an analog pin and resistor voltage divider to track the battery level.