kernel 5.9 is an option i could try but older kernel has known issues with the AMD graphic chip that is used for onboard and HDMI display, basically the display flickers with older kernel’s.
in my research the problem is here:
[ 1265.233] randr: failed to create shared pixmap [ 1265.233] (EE) NVIDIA(G0): Failed to configure Reverse PRIME on output DP-1-0.2.
this is due to an AMD driver bug:
The NVIDIA driver currently cannot be used as an output sink when the output source driver is xf86-video-amdgpu.
i have had some luck getting only the type-c dock-monitors working. but doing so, I lost the laptop on-screen and HDMI display 
here’s what I did to get the type-c dock monitor’s working:
Create /etc/X11/xorg.conf.d/10-render-offload.conf with contents:
Section "ServerLayout"
Identifier "layout"
Screen 0 "nvidia"
Inactive "amd"
EndSection
Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "PCI:1:0:0"
EndSection
Section "Screen"
Identifier "nvidia"
Device "nvidia"
Option "AllowEmptyInitialConfiguration"
EndSection
Section "Device"
Identifier "amd"
Driver "amdgpu"
BusID "PCI:5:0:0"
EndSection
Section "Screen"
Identifier "amd"
Device "amd"
EndSection
sadly the whole KDE desktop experience is slower than I would expect it to be.
i am no expert with the xorg config files, Looking online for a config/samples so that I can try re-writing the above with both AMD and nvidia driver (fingers crossed)