FWIW, My Z370 Taichi works with any sane setting (XMP, manual, tight, loose) at speeds 4000 and below, POST's first time every time. Any speed above 4000 using b-die 8GB sticks requires anywhere from 3-50 restarts to train memory. Once it POST's, it is rock solid every restart after that, unless you change some memory setting that requires retraining, then I have to do it all over again. Currently running 4266 XMP sticks at 4266 16-17-17-32 2T with aggressive tunes subtimings on taichi Z370, but it did require 30 restarts last time I changed settings for it to POST.
a high end board designed for high memory OC with only 2 ram slots will do higher speeds more reliably.
AMD Ryzen Threadripper 2950X
Asrock X399 Taichi
2x G.SKILL F4-4000C19D-32GTZSW (4x16GiB 4000MHz 19-19-19-39-2T)
EVGA NVIDIA GeForce GTX 1080 Ti FTW3
Seasonic PRIME 1300W 80+ Platinum