It's pixelated because you are using the ultrafast preset and a bit rate that is too low for that resolution/fps. Change the x264 preset to veryfast and either downscale your resolution or increase your bit rate to 3500.
Are you streaming to Twitch? If so, make sure you enable CBR and set your keyframe interval to 2 in Advanced options. Also, I recommend not using a custom buffer...just set your buffer to be the same as your max rate.