Have a look at the gpio-shutdown dtoverlay https://github.com/raspberrypi/linux/blob/rpi-4.19.y/arch/arm/boot/dts/overlays/readme#l692-l721
By shorting pin5 and pin6 on the GPIO header you can shutdown LibreELEC and if you then press the button again it'll power up again.
so long,
Hias