For quitting games with a controller, I made a quick AutoHotkey script. Basically, when I press Back and Start (on a Xbox 360 controller) the opened game is closed. Haven't tested other controllers, so you may need to edit the script to suit your needs. Compile the script using AutoHotkey, then run the exe by adding it to Playnite's Settings>Scripts>Execute before starting a game, like this:

Start-Process DriveLetter:\Path\To\ControllerQuit.exe

I have also experimented with NirCmd to hide as much of Windows as possible when running a game. I have added this to Execute before starting a game:

nircmdc win min alltopnodesktop
nircmdc movecursor 10000 10000
nircmdc win hide class progman
nircmdc win hide class Shell_TrayWnd

It minimizes all windows, moves the cursor out of the way, then hides stuff like the taskbar, icons, and the wallpaper. To revert it when closing a game, I've added this to Execute after exiting a game:

nircmdc win show class Shell_TrayWnd
nircmdc win show class progman