You could try to only disable the Intel GPU (see nixos-hardware/common/gpu/intel/disable.nix at 2eccff41bab80839b1d25b303b53d339fbb07087 · NixOS/nixos-hardware · GitHub).
boot.blacklistedKernelModules = [ "i915" ];
boot.kernelParams = [ "i915.modeset=0" ];
Other kernel parameters for your AMD GPU should not be required as that is done by the hardware.graphics option. Sorry if my initial answer was a bit confusing.