how to install nixos from legacy booting
first, boot via legacy, and install with the following options
boot.loader.grub.efiSupport = true;
boot.loader.grub.efiInstallAsRemovable = true;
boot.loader.grub.device = "nodev";then boot that via EFI, and change the config
boot.loader.grub.efiInstallAsRemovable = false;
boot.loader.efi.canTouchEfiVariables = true;then NIXOS_INSTALL_BOOTLOADER=1 nixos-rebuild switch and confirm nixos is listed in efibootmgr, then you can safely delete /boot/EFI/BOOT/BOOTX64.EFI