Eureka, looks like @joekamprad was correct in that it was either a CPU or RAM issue.
I iteratively stepped back my mother board settings.
- First I tried turning off TPM, sudo pacman -S nvidia-dkms, hard lock
- Then I tried turning off XMP, , sudo pacman -S nvidia-dkms, hard lock
- I restored optimized defaults on my motherboard which reset 2 CPU specific settings, 1 CPU/RAM setting.
- BCLK 100Mhz lock → Disabled
- Enhanced Turbo → Auto
- Undervolt Protection → Auto
With these off, sudo pacman -S nvidia-dkms, dkms / dracut successfully rebuilt and I could load into KDE.
So it was an “overclock” issue, I suspect Enhanced Turbo as this allows 2 cores to 5.8 and the rest to 5.5 on a 13900K.
This is a feature of Intel CPUs and shouldn’t cause this issue, I am going to turn each back on one by one to determine which is causing it.
EDIT It was/is Enhanced Turbo