"install image" invokes install-postinst-new which has a hardcoded reference to x86-64-efi:
mkdosfs -F 32 -s 1 -n EFI /dev/$part >&/dev/null mount /dev/$part $grub_root/boot/efi output=$(grub-install --no-floppy --recheck --target=x86_64-efi --force-extra-removable --root-directory=$grub_root --efi-directory=$grub_root/boot/efi --bootloader-id="$bootloader_name" --no-uefi-secure-boot 2>&1) umount $grub_root/boot/efi
For arm64 this needs to be set to --target=arm64-efi, so we need to change --target= to be correct for the system architecture.
The rest of the install script will work without modification.