GRUB i386 installing on AMD64 hardware

Relative newbie to NixOS. Assume I know nothing.
Installed NixOS on a Purism Librem Mini. I’ve got a number of issues. The one to address here is that by default, the installer opted for the i386 build of GRUB. The other, in case it’s related, is that I have to manually select which device to boot from using the BIOS (No UEFI) boot menu—otherwise SeaBIOS complains there is no bootable device.

CPU: quad core Intel Core i7-10510U (-MT MCP-) speed/min/max: 527/400/4900 MHz
Kernel: 6.6.83 x86_64

$ sudo eza -l /boot/grub
drwxr-xr-x    - root 19 Mar 19:26 fonts
.rw-r--r-- 6.3k root 22 Mar 21:08 grub.cfg
.rw-r--r-- 1.0k root 19 Mar 19:26 grubenv
drwxr-xr-x    - root 19 Mar 20:50 i386-pc
drwxr-xr-x    - root 19 Mar 20:50 locale
.rw-r--r--   56 root 19 Mar 19:26 state

Relevant config section below:

{ config, pkgs, ... }:
{
  imports =
    [ 
      ./hardware-configuration.nix
      ./modules/virtualization.nix
    ];

  nix.settings.experimental-features = [ "nix-command" "flakes" ];

  # Bootloader.
  boot.loader.grub = {
    enable = true;
    device = "/dev/sda";
    extraConfig = ''
    set debug=all
    '';
 };

...
}

Assistance greatly appreciated! If nothing else, I’d like to understand why i386 was chosen over 64-bit version.

I am also new here and I may mis-read but did you check this option

https://search.nixos.org/options?channel=unstable&show=boot.loader.grub.forcei686&from=0&size=50&sort=relevance&type=packages&query=boot.loader.grub