Keep android phone off until I press combination of volume + power
Your best and only bet is to root the phone. There may be an xposed package that will be able to give you the functionality you want. Look into the Magisk platform. I wasn't able to find a package for it but that does not mean it doesn't exist. 

As far as on startup or shut down, what you described is impossible. That's controlled by the phone bootloader and cannot be changed. Attempting to do so can destroy the phone permanently.

Note that any of these procedures will void your warranty.