Updates!
Was able to get two displays working with the onboard HD 530. Steps necessary in
this post (and see first post in the thread for complete instructions).
tl;dr: requires FB patch, 4-port patch, and hotplug from "outer" DP port once booted. But it works.
Voodoo audio works, however the internal speaker clicks loudly when booting until the driver sorts itself out. I disabled the internal speaker in BIOS (but kept the audio device) and that eliminates the clicking but allows audio.
Not sure if I mentioned it, but I did find a BIOS option to set the "Video Memory" size, so I don't have to patch minStolenSize in the Graphics kext.
Only remaining issue is front USB3 ports have disappeared. Installed macOS using them, but they don't register any longer.