For the time being I can use boot.kernelPackages = pkgs.linuxPackages_latest; in my configuration, and then set the kernel for a manual build to the same. Doing that allows me to use insmod on the package.
I’m running out of things to try at this point with regards to getting it to install properly on nixos-build. Is it possible using flakes + home-manager breaks something that prevents the install step from working?