NVENC delivers a very low-quality compression, requiring a LOT more bitrate to look on-par with an x264 stream at the same resolution and framerate.
As Harold said, switch to x264 (Veryfast). I'd disagree with downscaling further, 720p@30fps, 2000kbps is the 'golden point' for non-partnered streamers, using x264-VF.
If you wanted to stick with NVENC, you would indeed need to downscale further (while maintaining 2000kbps bitrate), to 480p or 360p.