Short-circuit in B550M Steel Legend£

I have had my B550M Stl.Lgd for about 24 months and it has constantly been plagued by random reboots. Mostly it is just flat-out system reset with no warning or blue screen or anything.
The past few weeks it had been giving a WHEA_UNCORRECTABLE_ERROR blue screen that would not calculate its %, I would always have to hard-reset manually when that happens.
I had suspect a short-circuit somewhere because every now and again when I plug in a USB pen drive in the rear, the whole thing shuts down, instantly. Not very often, but it happened enough to be noteworthy. This is when I got suspicious about the board.
On top of that, it would almost -always- crash/reset if I had Polychrome open for long enough. So I got pretty good at flipping it open, configuring quickly and closing it again. I do have a lot of RGB in here and so I thought maybe I had hooked something up incorrectly or crossed a wire.
But I have since disconnected all the RGB (unplugged the ARGB headers) and the issue continues.
Now on a recent crash my 2Tb PCI Gen4 SSD (installed in the under-the-Armor/GPU slot) didn't come back up again. It's no longer recognised in any slot, case or reader. It's fried. Not nice.

So I ordered a new SSD, thinking, well ok, maybe SSD issue, that's what was causing the crashes.

But no. The new SSD (different model) crashes also. Only 3 times so far, but that's in a week, so not exactly rare either.

There is one curiosity I have noticed, and that's that HWinfo reports wayward readings for the metric: Power Deviation Reporting Accuracy
At the moment it looks like this:
Current -- Minimum -- Maximum -- Average
64.4% -- 50% -- 141.3% -- 67.6%

For info the max CPU core temp at this point is 64°.

I have no overclock in the system; except that I am using 3600MHz RAM with resulting InfinityFabric matching, I guess. BUt I have also run the RAM at 2666 just to be sure and the reset still happens.

The specs are:
Ryzen 9 5950X
RTX 3090 FE (but had a 3080 originally and had the same issues)
64Gb 4x16Gb 3600Mhz, full-auto
Intel X520 10G NIC
2tb main M.2 under Armor/GPU + 4tb SSD in second/PCI-gen3
Lian Li O11D mini
and well 14x 120mm fans. :E
PSU is CoolerMaster V850 Gold SFX.
Also have Gelid Extender cables on both the GPU (2x) and 24pin ATX.

If I had a spare PSU, I would test swapping that out, but I don't, and anyway there is nothing SFX over 850 that is remotely good value so it would be cheaper to swap out the MB in that case. Though less convenient obviously.

So the question is are there any known issues with this board, any other reports of similar issues? Or any clues here that someone can recognize as potentially indicative?
Appreciate any thoughts.