Asus ROG Crosshair X670E Extreme suddenly stuck on Memory Test 00 - Advanced Troubleshooting Failed

Hey all,

I've been building PCs for 25 years now and this is the first time in probably 15 of those I've been completely stumped, so any help or ideas would be extremely appreciated. I've done a tonne of reading on what seems to be this relatively common issue, and tried all the troubleshooting I could find (detailed below the specs).

I built a new PC 1 month ago with no issues - working perfectly until last night. I did get this error when I first switched it on a month ago, but that was due to me being a dummy and seating the RAM in A1/B1 instead of A2/B2. I have not had a single problem, not even a PC crash, since. I was using the EXPO Profile in BIOS prior to this problem. I'm not running any other overclocks, voltage changes etc - hadn't got around to it yet this time.

The full specs are (with links to the seller for ease of reference):
Asus ROG Crosshair X670E Extreme
AMD Ryzen 9 7950X3D Processor
G.Skill Trident Z5 Neo RGB 64GB (2x32GB) 6000MHz CL32 DDR5 EXPO F5-6000J3238G32GX2-TZ5NR
ASUS ROG Strix GeForce RTX 4090 OC Edition, 24GB
Corsair iCUE H170i Elite LCD XT Display AIO CPU Cooler
Crucial T700 4TB M.2 2280 NVMe PCIe 5.0 SSD (Seated in the ROG Gen-z.2 to ensure available lanes on PCIE_1)
Corsair AX1600i Titanium Modular 1600W Power Supply
Cooler Master HAF 700 EVO ARGB Tempered Glass Case

Plus a bunch of SATA Storage Drives and a 2nd M.2 - Samsung 970 2TB in the Gen Z)

Troubleshooting Completed:

• Reseated RAM, many times
• Reseated CPU and Cooler (ensured the cooler wasn't too tight too, and booted without the cooler on the CPU briefly)
• Reseated GFX Card
• Changed RAM slot configuration; ran each stick through every DIMM (8 boots)
• Reset CMOS
• Attempted to boot from secondary BIOS on the motherboard (should be default as I haven't flashed that one)
• Flashed BIOS using the BIOS Flashback feature to the most recent version, released a month ago
• Attempted to boot with no RAM installed (weird it is still getting stuck on memory test!)
• Left the PC running at the Memory Test 00 phase for 6-12 hours multiple times at various troubleshooting points)
• Attempted LN2 mode
• Ensured all connections were firm and not loose - reconnected most of them to be sure too
• If I had other DDR5 RAM, I'd swap that in temporarily, but unfortunately I don't have access to any

I am completely out of ideas. If any piece of hardware is stuffed I think it would have to be the motherboard or RAM... but it's extremely weird that it just suddenly wouldn't boot up after a month of constant use, excellent performance and no problems. I think it did a Windows Update last night but that's all that would have changed - and that doesn't exactly affect POSTing.

Anyone got any ideas for something I should try?