Oof okay so I've had a stable system for a few months now, but after updating to 10.15.1 (and your latest EFI, copying over serial, UUID, and board serial), I am getting some issues.
iGPU is disabled, I have added the agdpmod=pikera flag (that's from memory but the boot flag is copy and pasted from WEG github so it's correct)
Every couple of boots, I get a black screen on my main monitor.
On every boot, the first login screen (filevault?) has a checker-boarded background image. The UI elements are not affected.
But here's the killer. Whenever I play a video (VLC or IINA, not youtube on firefox), I get really bad graphical artifacts. It's where part of the moving image sticks and pixelates (as if you had a bad antenna on a TV.) After about 10 seconds of this, both of my screens go black and my fans go on full blast. The system is still up (there are no crash reports and I can still hear the video playing) but it is no longer usable until I restart.
Again, latest EFI..
Any ideas? I am a software dev so I can do whatever troubleshooting you need; however, I should note that I am not very familiar with debugging macs.