hide grub on computer start

In /etc/default/grub set:

GRUB_HIDDEN_TIMEOUT="1"
GRUB_HIDDEN_TIMEOUT_QUIET="true"
#GRUB_TIMEOUT="1"

That will allow you to have a very short time interval of 1 second to still press Shift while booting to get the advanced menu, while not running into the 10-second problem in

if [ "${timeout}" = 0 ]; then
set timeout=10
fi

in /boot/grub/grub.cfg.

(Meaning, "0" is allowed, but just gets set to "10" in a deeper level of the config to protect users from themselves)