You have tested different USB-C to HDMI/DisplayPort cables... do you know if one of those cables has support for "DP Alternate Mode"? That should work... also, ajusting Mirroring, Refresh rate etc...
Give me a few examples of your cables (brand names and model numbers)
I'm using the GTR5 5900HX and my non expensive USBC-2-HDMI-DP adapter works just fine.