You may (or may not) know that most Nvidia cards have a limit to the number of NVENC encoding sessions you can run at a time. Ever tried to have Vegas encode more than 2 videos using NVENC at a time? Ever got a weird error about resources? Thats because Nvidia cards limit the number of NVENC sessions you can run at a time. But this limit is a software one. If you have a Quadro card your sessions are unlimited and most GTX and RTX cards use the same architecture and chips as the Quadro cards.
Here is a handy link showing the available processors and session limits https://developer.nvidia.com/video-encode-decode-gpu-support-matrix#Encoder
Here is a handy patcher that will remove that limit. I have tested this on my own RTX 2060 and can now render multiple projects at a time.
I contacted the developer yesterday and he just added a patch for the latest 419.67 CRD drivers. They have patches for versions 417.35 through to 419.67.
THE KICK IN THE NUTS
Now, dont get excited thinking that this will solve your problem where you render and its only using 20% of your GPU. That is still the case. This just lets you run more rendering sessions at once and make more use of that spare processing power.