I would suggest making sure that the firmware within the Dell D6000 dock is up to date. The only issue with that is that you will need a Windows system in order to do the update --> https://www.dell.com/support/home/en...-d6000/drivers
This actually worked! I had to boot up a Windows PC *just to run the utility*, but it worked and now I'm back on macOS with my HDMI display *and* my DisplayPort display working again. Thanks!