I have the Z270-P as well, I'm running SimpleMiningOS with 6x GPUs.  I had an issue as well with using the first PCIEx16 slot.  5 GPUs were easy to get up and running, but it took a while to get that 6th one up and running.  I'm using the latest bios downloaded and installed through the Asus flash utility via internet.  I have 4g Encoding Enabled, and the other options set to Gen 2 (I've heard Gen1 works better for Windows, not 100%).  Other thing was that even if I set the onboard video to primary, it sends the video feed to the card plugged into the first PCIEx16 slot.  Make sure your monitor is plugged into this card.