I am using Prime Z270-As. I don’t have this issue. Most are running 7 cards right now. Some of your setting are different than I am using. I would suggest starting with this video:
Here is the one for the Z270-A. Use this as a guide
This guy’s videos rock. Take a look at his Ethereum HDD build out. He is also the reason why I now have an Orico SSD cloner. SO much faster to set up computers now.