She'll be alright, indeed.
I took the board out to my usual computer shop to have it tested for a Third Opinion, and the guys caught something I hadn't: The board was identifying itself as an X570 Phantom Gaming
ITX
/TB3. Aka The Rainbow Warrior.
I had accidentally flashed in the UEFI meant for The Rainbow Warrior instead of the one for the Phantom Gaming X. Whoops.
This kinda explains the bad temperature and fan reading and inability to support more RAM, but it is still perplexing - both this board and the ITX/TB3 has a Nuvoton sensor chip (not common on AMD platforms, but apparently Asrock prefers it to ITE chips). In theory temperature sensor should still work properly.
So they downloaded the correct BIOS onto a USB drive for me and tried to reflash the BIOS.
Except the board wouldn't have it.
So I convinced them to rename the file to creative.rom and use the way that got me into this mess- by using BIOS Flashback.
And it worked. Sensors are back to normal and even the RAM issue is gone.
The only downside is one of the NVMe slots still appear dead. Except that I got the gut feeling to swap out the MP600 in that slot and replace it with the last NV2 I have hanging around.
And that totally worked. The slot now reads both a single MP600 and a NV2. So somehow I have killed a NVMe Drive in the process. No big deal I guess.
So yeah. My cache is down to 256GB. My main OS drive remains at 512GB.
Currently it's running Memtest86+ on my desk. But we are in the home stretch again.
I also took the opportunity to fix up some errors I made the last time and improve my cable management. I remembered the expansion slot cover this time (although getting the GPU in with the cover in place is a real PITA).
But yeah, we're almost ready to end this.
However this experience serves to highlight the dangers of BIOS Flashback: apparently zero validation is done to make sure the BIOS is meant for this board. I could've tricked it to flash the BIOS for a Gigabyte X570 Arous Master and it would have obliged. In the wrong hands this opens the tool up for the Evil Maid attack, which a "friend" can flash your PC with a tainted BIOS or even brick it for the evulz. Asrock should investigate this as this dangerous.
Addendum: remaining MP600 just died 4 hours later. Looks like those drives were on the way out for some reason. Welp, I need 9 drives, not 8, now. Also, machine has been rolled back to the point where all the expansion cards are removed.
Addendum 2: I'm also not happy with the SSD cache size.
Addendum 3: Further research indicates that this is a common problem with first gen MP600, that they suddenly would drop dead after just months of use mysteriously. Why did I even...
Round 2
Memtest86+ is working