Managed to get 4133MHz working reliably. didn't touch VCCSA or VCCIO. They're on auto for now. yes, I know, auto sends a lot of volts. I'll work on that later. Currently testing 4133MHz CAS15 with tightened RTL/IO-L's (Planning to be 24/7). Will work on 3rd timings (seems to most strongly impact bandwidth) > primary timings > TREFI + TRFC > rest of secondary timings > final polish. 4000MHz CAS14 is "stable" too (throws an error once every 6 hours or so on memtest86 8.1 pro). Anything past 4133 seems out of reach (Dual rank modules). POST code wildly loops/flickers after a faster than normal 30 > 32 > 35, sometimes landing on 23, rarely on 55; I know 55 is almost definitely RAM failed to train (bad timings). 7F seems to also be RAM training related but less often. 23 is just... hard to diagnose for me so far.
tWTR settings (tWTR, tWTR_S, and tWTR_L) all don't seem to do anything. Can anyone confirm? "tWTR" seems to be a misnomer as same-bank tWTR_S and different bank tWTR_L are their own timings, detected to be 6 and 1 for L and S respectively, regardless of whatever you put in UEFI settings.
Speaking about XI APEX board, in case it wasn't known.
Intel Core i9-9900k (Silicon Lottery 5.1GHz @ 5.3GHz 1.36 VCore)
ASUS ROG MAXIMUS XI APEX
G.SKILL F4-4000C19D-32GTZSW (2x16GiB 4000MHz 19-19-19-39 2T, tuned to 4133MHz 14-17-17-31 2T)
EVGA NVIDIA GeForce GTX 1080 Ti FTW3
Seasonic PRIME 1300W 80+ Platinum