My first thought was power issue since it started with "PC Shut off on its own", but I see you tried 2 different PSUs. Did that include different PSU cables? And a different wall socket, in case there is a grounding issue with the wall socket?
Speaking of cables, is the OS drive an M.2 or SATA? If SATA, did you try different cables and/or ports? Different power cable/psu for the SATA drive?
With all drives disconnected, can you boot from a bootable USB stick?
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Z690-E, i9-12900k, 2x16 DDR5, 980pro nvme x3, Ryujin-II 360, Strix Animate-II, Evga 3090 ftw3