A Dell Optiplex has a BIOS setting to boot on power on. You can use a single mains switch to control a regular powerstrip which powers on everything including the pc....
Just let the FrontEnd shut down Windows...
Today I thought about start-up/shut down and came to the same arrangement.
Under my flip-up control panel I will have a mains power switch, and switches for the marquee lighting. Also the amplifier front panel will be accessible (power, volume). This will let me power off the PC and amp, but leave the marquee lit up for mood lighting

Also under there will be the only button to exit the frontend.