Am I Trying to do Something Stupid?
@tpa77l3, Those cables don't always work bidirectionally because the monitors may or may not support that feature.
Those cables don't always work bidirectionally because they are not bidirectional.
Some USB-C to DisplayPort cables are bidirectional (like the one from Moshi or Alogic). Bi-directional means you can also use it with DisplayPort output of GPU to USB-C input of display. The cable you linked did not mention bidirectional capability.
USB-C to HDMI cables can only work in one direction -> USB-C output from GPU to HDMI input of display. If you want to use HDMI output of GPU, then you need a HDMI to DisplayPort converter combined with a bidirectional USB-C to DisplayPort cable.
Not all displays with a USB-C port will work with a bi-directional cable that does not include a USB signal. Some displays with that problem may work with a subset of those cables (for example, the Alogic cable works with the 22 inch LG UltraFine 4K display but the Moshi does not).