The openSUSE guide said most Debian systems use ext4 for /boot, copied
from the Debian guide. openSUSE uses Btrfs by default, so say so and
point to the Btrfs driver, keeping ext4 as the example.
The ARM64 step copied the arm64-efi GRUB modules into
/boot/grub2/x86_64-efi, where GRUB on ARM64 doesn't look for them, so
insmod chain and insmod fat failed.
The unsigned setup guides passed the EFI System Partition device to
efibootmgr -d, which expects the disk, and left out -p, which defaults
to partition 1. The command failed, or created a boot entry for the
wrong partition when the ESP isn't the first one. Pass the disk and the
partition number, as the signed guides do.
alex
Robert Krátký