Hi,
I have 2x16GB 4000MHz CL14 RAM kits.
So far, only 0707 BIOS version lets me XMP with 12900KS.
With another BIOS version, I only succeeded with 3800MHz, by proceeding the same way as OP (enabling XMP, decreasing to 3800MHz, decreasing voltage).
With the last BIOS version from two days ago, I couldn't even boot with 3600MHz.
Ok I am very far from being knowledgeable when it comes to computer hardware, but I would have imagined those RAM kits were manufactured and sold for a reason. It really is not transparent what piece of hardware is compatible with what piece of hardware.
So I'm just using the 0707 BIOS version which was released in October.
I'm happy my RAM is fully utilized using XMP2.
Now I'm concerned with taking maximum benefits from my 12900KS, right now with stock settings the temperature is about 55-63°C, I think it's quite low (I have a good cooler, Corsair iCue H170i LCD Elite - 420 mm).