Minimal way to always have screen on on Android?
Set the 'keep awake while plugged in' setting
(Screen will never sleep while charging)  

Settings - Developer options - Stay awake: on  

or from terminal  

svc power stayon true


if the user can press the power button, set the screen lock to "none" (not even swipe) and
use Tasker/MacroDroid/Automate to watch the power button. or run a watchdog from shell  

the following example depends on three android binaries
/system/bin/toolbox
/system/bin/toybox
/system/bin/input  

# (replace first line with while 1)
until [ $(($(toybox cat /sys/class/power_supply/ac/online)+$(toybox cat /sys/class/power_supply/*usb/online))) -eq 0 ]
  do
    case "$(toolbox getevent -lc4 /dev/input/event2 | toybox head -n1)" in
      *KEY_POWER*)
        input keyevent KEYCODE_WAKEUP
      ;;
    esac
done  


the above code checks for charger is connected as entry point for the loop. the loop exit when charger is disconnected. /dev/input/event2 may vary for your device. you can check which input event has only 3x buttons [Vol+|Vol-|Pwr] from getevent -il (for more generic code look here)