A small, perhaps surprising update - applying a SSDT (attached) i'm able to use a thunderbolt monitor successfuly.
Although its locked at 1440p, it works! From passed through GPU i have dp -> mini dp into thunderbolt card that is passed through to macos and then I have thunderbolt cable in to thunderbolt monitor.
One thing that's not working is thunderbolt usb dock. no matter what I do I can't get the devices plugged in the dock to appear. Or the dock itself to appear. However, I was able to put the dock between the monitor and the thunderbolt card and i also get signal. No USB devices though. Weird.
Any thoughts? My IOREG now (without anything plugged in looks like attached)
Also, the monitor only works when plugged into one of the two ports.
SSDT-TB3-HackinDROM2.aml.zip