GPU can be used in several tasks: decoding, frames calculating, rendering.
In SVP menu GPU-acceleration you are selecting device for frames calculating.
See more: https://www.svp-team.com/wiki/GPU_Compatibility
Your discrete graphics Radeon RX 560X is powerful enough to SVP task. So you will get GPU load far from 100%. Is it what you expect?
You can disable GPU-acceleration in SVP and enable it again to get difference.
I think your Vega 8 is used more than RX because of it used for rendering.
You need to move rendering work to your discrete GPU. To do this disconnect your display from MoBo and connect it to Radeon RX 560X output.