Welcome to the ROG forum !
You say your ram works fine in your z270 board, is this with the same cpu? Since they are different boards they could be applying different voltages.
It sounds like maybe the IMC in your cpu has weakened a bit.
Try raisng both the CPU VCCIO voltage and CPU System Agent voltage to 1.15v - 1.20v and see if this gets you stable.
The cpu's IMC degrading is kind of what it sounds like according to the description of your problem.
I would have the Dram voltage at no less than 1.35v and I would try it up to 1.40v. It's safe to do so, Intel recommends no more than 1.50v for DDR4.
Does this get your ram stable ?
It's possible it could be the board.
Have you tried resetting to defaults and setting the ram manually with no cpu overclock ?
If you can't get it going I'll suggest to contact ASUS Tech Support, they will likely have you try some things and/or issue you an RMA. ASUS has 24/7 phone support here in the U.S.