I'm having trouble getting a truly headless (no graphics card) running AMD64 Testing to boot to serial console.

I think I got the installer to run by adding

console=tty1 console=ttyS0,115200n8

to the installer's boot incantation and successfully installed Bullseye via the serial console. I accepted the default install options.

Upon rebooting, I see the GRUB2 menu. I can edit the grub2 menu item. However, the same ttySO incantation above doesn't work when booting the installed system.

I was hoping to be able to boot the thing into single user mode to fix GRUB, but unable to get that far and most references to booting to serial are many years old.

Thoughts?

So if it works and boots to graphics mode?

Try; putting an # in front of whatever displays listed in: /etc/X11/default-display-manager

...then reboot.

I do not have hardware to check, but I used serial port of my VirtualBox and added
to my "linux" section in grub boot menu.
It perfectly works.

Checked on 10 (buster).
Try removing "quiet" from kernel commandline, check output carefully. Do you see output on your serial device?
If no, that even /dev/console is not connected.
Check your "dmesg" to make sure kernel knows about your serial port, make sure speed is also correct.

>ttySO
ttyS0 Zero, not "O".

Also, boot normally and check that serial line works.