Which GPU is the Xorg server running on? (If nvidia-smi shows Xorg, then it’s the nvidia, otherwise the amd).
Note, that
The NVIDIA driver currently cannot be used as an output sink when the output source driver is xf86-video-amdgpu.
(source)
This means that the Xorg server must be running on the nvidia gpu if you want to use external displays.
Follow this guide but leave out the first step, do the following instead:
pamac build optimus-manager-git
and also omit the seventh step. After completing the steps, run optimus-manager --switch nvidia and hopefully it’ll work.