grub.cfg is missing a command for terminal_input, so when a serial console is enabled, the input is taken only from the graphic console. The result is that the grub menu is seen but doesn't respond to keystrokes.

The command added should be:

terminal_input serial console

As detailed in: https://www.gnu.org/software/grub/manual/grub/html_node/Serial-terminal.html

In this case, a terminal in which you press any key will be selected as a GRUB terminal.

The first terminal that receives a keystroke will be selected so this shouldn't break the graphical menu.

The top of grub.cfg thus looks like:

set default=1
set timeout=5
serial --unit=0 --speed=115200
terminal_output --append serial
terminal_input serial console