OK, so we know that the lack of power during GPU spikes was not the issue, but it's still wise to have upgraded the PSU.
The two dumps both point very clearly at either the graphics driver or the graphics card...
One is a 0x116 VIDEO_TDR_FAILURE bugcheck, this happens when the Windows Timeout Detection and Recovery feature (TDR), which detects a graphics hang and resets the graphics driver and graphics card, fails to recover from the hang. The cause here is almost certainly either the driver or the card.
The other dump is a 0x133 DPC_WATCHDOG_TIMEOUT. A DPC is a Deferred Procedure Call and they are typically used in the back-end of device interrupt processing, the DPC code is part of the device driver. In this dump the graphics driver (nvlddmkm.sys) is where the failure happens...
Code:
FAILURE_BUCKET_ID: 0x133_ISR_nvlddmkm!unknown_function
Note that this failure bucket blames the ISR, the Interrupt Service Routine, which is the front-end of device interrupt processing, the ISR code is also part of the device driver. Long running of either the ISR or the DPC will cause this 0x133 bugcheck.
You thus have two dumps here, both pointing very clearly at a graphics problem, just as the earlier dump did. We now know it's not a power problem, although be sure that the additional power cable is securely plugged into the 4090 card and the PSU.
The first thing I'd suggest is that you remove the 4090 card and then re-seat it firmly. You'd be surprised how many times this simple action solves the problem. The slightest bit of dust or dirt between a card pin and the slot can cause all sorts of issues.
If that doesn't help then your next best option is to remove the 4090 and plug the monitor into the motherboard port and use the Radeon graphics iGPU on the CPU and see whether it crashes or BSODs then. If it's stable without the 4090 installed then you know for certain that the problem does lie with the 4090 or the driver. You might combine this test with removing and re-seating the 4090.
The best way to check whether the 4090 or the driver is at fault is to download the four most recent driver versions for that card from the
Nvidia website. Also download
DDU. Use DDU to uninstall the existing driver and then manually install the most recent driver. If it crashes or BSODs use DDU again to remove that driver and then manually install the next most recent driver. Keep doing this until you either find a driver where it's stable or it BSODs/crashes on every driver version. If it fails on the four most recent driver versions then the problem is likely the 4090 card.