i9-14900K + Z790 Dark Hero unstable with DDR5-8000 CL38 IMC, motherboard or BIOS settings? Hi, I’m trying to stabilize my RAM setup and I’d appreciate some advice.

Intel Core i9-14900K — stock, no CPU OC/undervolt
ASUS ROG Maximus Z790 Dark Hero
G.Skill Trident Z5 RGB 32GB (2x16GB)
DDR5-8000 CL38-48-48-128
Kit: F5-8000J3848H16GX2-TZ5RS
XMP II
At first, I ran the RAM at 8000 MT/s without manually changing timings and I could game for around 3 hours without any noticeable crashes.
Later I started changing/tightening memory timings, and since then I’ve been having stability issues.
I went back to roughly the XMP settings and tested:
8000 MT/s: OCCT Memory crashes after ~30-40 seconds
7600 MT/s: OCCT crashes after ~8m40s with Code -1
TM5 Universal 2 at 7600 MT/s: 1 error after ~1m15s
Currently testing 7200 MT/s
Current voltages/settings were approximately:
SA: 1.20V
IVR TX VDDQ: 1.35V
Memory Controller Voltage: 1.40V
DRAM VDD: 1.45V
DRAM VDDQ: 1.45V
Command Rate: 2N
XMP II
The CPU has never been manually overclocked or undervolted.
My main question is: does this look more like an IMC limitation on my 14900K, a limitation of the Dark Hero’s 4-DIMM memory topology, or simply incorrect memory voltages/training/timings?
Since the system previously seemed fine at 8000 during gaming, I’m wondering if changing the timings caused bad training/settings that I haven’t fully reset.
What would you recommend testing first to determine whether 7800/8000 can realistically be made fully stable on this setup?