I've probably parsed dozens of pages now, including the "Dual boot NixOS and Windows" page on nixos.wiki, and not really sure what the best steps are since most seem to leverage the fact that everything is on a single partition. My windows lives on a physically separate drive than NixOS, so osprober does not detect the windows partition at all. I tried to go down the route of grub-mkconfig but that doesn't seem to be a nix package and I couldn't mount my Windows bootloader as it is NTFS. Is this even possible with this configuration?

My next step was going to be to physically disconnect each of my disks/NVME, nuke everything bit by bit, then only connect the disks I want and install each OS with it's specific disk connected.