brado I'm 90% sure it's refresh-rate related.
Sound is likely also over HDMI.
Just lower the refresh rate of SteamVR.
Power should not be an issue, assuming your monitors have their own power supply. But as you said, it works WMR.
The interface of DeoVR (Unity) also has to be rendered aside from your monitors and VR video.
You could try to force the interface to run on your IGFX, I'm not sure that will fix it though, or could even create other issues. But it's always worth a try, you can undo it whenever you want.
Like I run my browser on my IGFX (which display doesn't matter. Doesn't even need a display attached.)
Settings > Display > Graphics Settings > Desktop app > Browse > \Steam\steamapps\common\DeoVR Video Player\DeoVR.exe > Pick prefered GPU.
This shouldn't change which GPU decoder is used! That's set in the codec settings. Only the rendering of the app is changed.
This can also help in game performance if you change your background apps from GPU.
Another solution could be using QuickSync decoder for the video. Then you are running the video on the IGFX. Though the Nvidea decoder is 'better'.