I'm glad to hear that it's working! I used a few hours myself to figure it out! Btw, I have "PCIe ACS override" set to disabled.
Regarding GUI:
I don't use unraid in GUI mode, only commandline, however you should connect you monitor to the VGA output on the supermicro motherboard, this will use the internal GPU and everything should be working. I'm using VGA on the motherboard for unraid and displayport on the P400 for Ubuntu. It's connected for troubleshooting only. I mostly work via PuTTy, TeamViewer/RDP.