I dont think in this case ASUS has any fault.
First, you say yourself, that gaming is fine, no issues.
Second, the shop and ASUS tested your card and found no issue.
Third, your fan curve is not appropiate.
I think your anger is misdirected.
I have G.Skill Trident Z5 5600C36 with 12900K/Asus Z690 Hero, works perfectly. I went safe as this runs at 1.2V while the 6000 is 1.35V.
DDR4 is a more reasonable choice, but if you don't upgrade often then maybe DDR5 is a better option.
I have a 1000W PSU but my UPS is 900W max. Take out the 3 monitors and network gear hooked up to it and I have about 750W left for the computer.
12900K system without video card maxes out at 300W, so 450W is left for the video card. Hopefully...