Unable to use external monitors on Ubuntu 20.04 with eGPU 1080Ti

I’ve spent the better part of the week around this before I stumbled upon this wonderful forum and I hope you can help me here.
I’m running Ubuntu 20.04 on a Dell XPS 15 (1650 hybrid) with a HP Omen running 1080Ti as a docking station.
The setup works almost flawlessly - all peripherals are identified, and both cards show up in nvidia-smi. I’m running prime-select on nvidia mode.
Under nvidia-settings, all 3 external monitors are identified correctly, albeit they’re disabled by default. Attempting to enable them by setting each to a different X session results, most often, in no working displays at all (not even the laptop display) - I’m guessing this is due to manipulating the xorg.conf where it shouldn’t exist?

tl;dr - with everything correctly identified, why can’t I use my external monitors? What am I missing?
nvidia-bug-report.log.gz (533.0 KB)