finding # CONFIG_HZ_100 is not set in .config was helpful
nixos-rebuild has been running appearingly successfully for a little while (still completing insanely long nixos kernel build) after setting, for CONFIG_KVM and CONFIG_KVM_AMD:
extraConfig = "KVM y
KVM_AMD m";
so I believe the main problem besides several syntax errors while adapting to boot.kernelPatches.extraStructuredConfig was that I had to set KVM_AMD to either "KVM_AMD m" in boot.kernelPatches.extraConfig or {KVM_AMD = lib.kernel.module;} in boot.kernelPatches.structuredExtraConfig.
So it had to be lib.kernel.module or m instead of yes or y.
edit: still building so I’ll have to test to ensure svm.c actually got built