Hey Casey,
Thanks again for all your work here.
Sadly, I got nowhere with MOD3 or MOD4 either. Thunderbolt simply didn't show at all. I've reflashed the original firmware and am back to the SSDT method, though to be honest - while it is nice having Thunderbolt bus. I think I am going to return to no SSDT at all. With no SSDT, I get Thunderbolt on every boot after the first warm boot, and it is stable. With the TbtOnPch SSDT I get random disconnects. It works some boots and doesn't work other boots, etc.
I think we can conclude that the GA-Z170X-UD5 TH is simply not great for Thunderbolt compatibility at this point, which is hopefully useful info for your repositories anyways.
Thanks again for experimenting with me!
EDIT: There's nothing regarding SMBIOS on this is there? I'm currently using iMac 18,3