If you also have fast moving details in a majority of your shot, 5500kbit/s is very low for 1080p 60fps, so maybe do a test recording with 15.000kbit/s just to make sure, that the pixelation is not simply based on compression.
When you downscale the webcam in you scene, you can also rightclick the webcam in your source list and change the scaling filter from none to lanczos. Although there might be already a lanczos scaling filter active in the video tab, I personally prefere to additionally have that dedicated scaling filter onto my webcam.