Update on boot issues with Asus CH6 and 3900X.
After further triaging I was getting random error codes (mostly 8 or is it B) during warm resets. My PC would rarely do a restart from Windows and any reboots required a hard shutdown and power off at the wall for around 15 seconds to be reasonably sure it would boot.
It turns out my issue was having CSM (Compatibility Support Module) disabled in BIOS. I have an M2 NVMe and had CSM disabled to speed up the boot times. This worked fin on my 2700X but the 3900X clearly doesn't like it.
So if anyone is having random error codes during soft (and sometimes cold) reboots with their CH6 and 3x00X please try CSM enabled in your BIOS.
I am now going to create a fresh windows install on s spare SSD before doing my DDR OC testing. I learned never to test any CPU or DDR OC on your actual Windows install as it
will corrupt when your OC inevitably fails.