If you want to add an initrd, create an EFI payload: https://wiki.archlinux.org/title/Unified_kernel_image#Manual...

$ stub_line=$(objdump -h "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" | tail -2 | head -1)

$ stub_size=0x$(echo "$stub_line" | awk '{print $3}')

$ stub_offs=0x$(echo "$stub_line" | awk '{print $4}')

$ osrel_offs=$((stub_size + stub_offs))

$ cmdline_offs=$((osrel_offs + $(stat -c%s "/usr/lib/os-release")))

$ splash_offs=$((cmdline_offs + $(stat -c%s "/etc/kernel/cmdline")))

$ linux_offs=$((splash_offs + $(stat -c%s "/usr/share/systemd/bootctl/splash-arch.bmp")))

$ initrd_offs=$((linux_offs + $(stat -c%s "vmlinuz-file")))

$ objcopy \

    --add-section .osrel="/usr/lib/os-release" --change-section-vma .osrel=$(printf 0x%x $osrel_offs) \

    --add-section .cmdline="/etc/kernel/cmdline" \

    --change-section-vma .cmdline=$(printf 0x%x $cmdline_offs) \

    --add-section .splash="/usr/share/systemd/bootctl/splash-arch.bmp" \

    --change-section-vma .splash=$(printf 0x%x $splash_offs) \

    --add-section .linux="vmlinuz-file" \

    --change-section-vma .linux=$(printf 0x%x $linux_offs) \

    --add-section .initrd="initrd-file" \

    --change-section-vma .initrd=$(printf 0x%x $initrd_offs) \

    "/usr/lib/systemd/boot/efi/linuxx64.efi.stub" "linux.efi"

The resulting linux.efi" can be added directly with efibootmgr, and contains the kernel boot parameters (cmdline)