Hi guys i have same motherboard z270-p with 5 rx 570 8gb & 1 rx 580 8gb
???my problem is that when i boot my rig with 5 cards without connecting riser in 2nd pciex16 slot i can go till desktop, but when i plug in the 6th card in 2nd pciex16 slot it keeps on restarting before i see windows screen or gives a blank screen.
I tried many bios combination from this thread please help will reward

Try this.
Boot to BIOS > Disable 4g encoding > Install 3 cards 2 x RX570 and 1 x RX580 with drivers. RX570 would need atidmag patcher.
Boot to BIOS > Enable 4g encoding > Install rest of RX570. You would see the dev manager might have market rest of the cards as unsigned. Sign again with atidmag.