Apologize for my bad English, I have a nearly identical system as yours, mine has 64GB of ram and a 4090 and I too have been getting these weird random shutdowns. I did some digging and found out it was the Seasonic Tx1600 because when I swapped an 8-year-old corsair rm1000x everything runs perfectly. I RMAed my TX1600, Seasonic tested in the factory and everything checked out, and they told me it could be that there is an incompatibility issue with the Z790 hero and the tx1600, they send me a replacement with a newer manufactured date which was made in March 2023 (my old one was made in Dec 2022), and I have been using it for a week now with no random shutdowns. People at Seasonic told me they have been working with Asus to improve the incompatibility issue. So if u decide to go with Seasonic tx1600 with Asus z790 hero please try to get one that is newly made, I don't know if it will make the situation go away, but it will at least help. I will keep on using my tx1600 and will post any updates that I have.