Just curious, did you have any issues getting the mobo to run fine with a riser in the main x16 slot? I have the Z270-P and I can't get it to run with a card attached to the PCIEX16_1 with a riser. I tried it with just 3 cards and it would only detect 2 cards, however I'm running 5 cards just fine by skipping that slot.
All three PCIEX16 are working ok with risers, never had any issue with it, though I previously had to RMA a similar mobo as I had a PCIEX1 slot not working at all