I have tried to hide the Grub boot with Fedora 34 with the following commands:
sudo grub2-editenv - set menu_auto_hide=1
sudo grub2-mkconfig -o /etc/grub2-efi.cfg
But nothing happens, the Grub Boot menu keeps showing up with a 5 sec countdown, I have only one kernel 5.14.9. Any advise how to completely hide Grub at boot.
I think this will help you. Even though it is for Ubuntu. This is applicable in all Linux systems.
While reading today I tested what you posted and it worked for me with 5.14.7 F35.
For legacy you might have to use sudo grub2-mkconfig -o /etc/grub2.cfg to write it back, or try to this before you try your string again.
sudo grub2-mkconfig -o /etc/grub2.cfg