You're using NVENC encoding for 1080p and only giving it 5500kbps to work with. Unfortunately, this is going to result in blockiness -- there's no getting around the bitrate needed to have good quality 1080p.
You can improve slightly by selecting "High quality" for your encoder preset -- this will change it to 2-pass encoding (Quality now reverts back down to 1-pass). There's not really much you can do past that.
If you really need better quality for 1080p at 5500kbps, the only option you have is to use x264 encoding, which is pretty much out of the question on a 2600k.
Your only other options are to either raise your bitrate, lower your resolution, or consider 30fps.
As an alternative, you could consider upgrading to RTX (or possibly upcoming GTX) for the Turing NVenc encoder, but that's still going to be a marginal quality boost. Nothing earth shattering.
