Well, couple more ideas for you. not sure if that warning you're seeing is the culprit or just a symptom though.
Do you have fast boot or "fast startup" enabled in your BIOS? You should disable that.
I feel like it has to be something related to your BIOS reset. So the fast boot would be one thing that some boards default to on that can cause problems.
Can you take pictures of your BIOS screens for CPU settings, Memory settings, and Boot settings (how your UEFI/Legacy/CSM settings are)? I know you've tried all sorts of stuff and we're stabbing in the dark at this point, but I'm hoping some setting got messed up and it sticks out if we can take a look around.
One other thing if you could, is in HWiNFO's sensors window, if you let your system run for a couple minutes (if it doesn't freeze on you), then take a screenshot of the CPU's core temperatures, as well scroll down and look at the hard drive sections and see if it gives you any status on how much life is left in your SSD and what temperature the drives are running at. I'm just looking for more clues at this point.