From that description it's definitely VSOC. Benching at 2500MHz (hell I'd say 2350MHz you're pushing it) is probably only possible if you don't use the iGPU. TM5 *can* catch IF errors once in a while but don't count on it. Frankly the iGPU is your best IF stress test, nothing comes close
My B550I Aorus Pro AX has always had a problem with any Renoir and Cezanne APU in that its LLC implementation sucks ass, and as soon as >10A of current flows through the SOC domain (ie. any significant iGPU load), VSOC drops like a brick. When that happens and VSOC isn't sufficient to sustain the IF at given speed, then instant reboot, no BSOD. Not sure if your Steel Legend suffers the same problem, you'd have to watch HWInfo.
Solution is just to set higher VSOC to raise the vdroop floor a bit. I'm at 1.256V SOC on 2300 iGP/2166 IF because it droops from 1.256V to 1.17V in game......I know someone who runs north of 1.3V SOC to sustain 2400/2200
The problem with trying to get vendors to try and fix something like this is that the only people who will have this issue are those that seriously use the iGPU. Anything (even Prime95 Large FFT or TM5/HCI) pales in comparison to iGPU usage in terms of VSOC current.