Thanks for the update on this. First thing I do when a system crashes while running Prime95, I send it back. First thing I do when I see CPU has ES bit set and I didn't order that, I send it back. Because there will never be any microcode updates for that ES CPU and who knows what bugs the stepping has. Cooking the m.2 on top of that just creams my corn.
If it HAS to be stable, imho there is just no way around tier 1 motherboard shops like OEM (Dell, HPE, Lenovo), Supermicro, Asrock Rack, maybe Tyan. The only small x86 embedded shop I ever saw that produces stable platforms is PCengines from Switzerland. Their Alix line with Geode CPU kept on running well into its teenage years. I expect nothing less from the more recent APU2 line.
Here is such an example "stable" system, a fanless APU4D4 (1 GHz AMD Jaguar, 4 cores, 4 GB ECC RAM, 4x Gigabit Intel i211), 512 GB mSATA SSD, Dell aka Sierra Wireless AirPrime MC7455 4G card on mPCI to mini-PCIe adapter (middle), Atheros 802.11n AR9590 Wifi card (left), Bluetooth dongle (not visible, on back USB), GPS antenna (not visible to the right):
To run nicely, I patched the Linux kernel and assorted software, i.e. kernel 5.4 +wireguard +ath9k no regulatory limits, no power limits, no antenna reduction (this Atheros card was once integrated in a system with big antennas, tx power is therefore more limited than with the usual USA reg domain cards), no 20 MHz fallback with wifi neighbors, +gpio_keys_polled fix +pcengines-apuv2 fix to allow ACPI enumeration for LED control, but also make front button scan code work (reboot). Also managed to make gpsd for chrony time service precise enough without PPS just GPS NMEA, so that it is now usable as a sub-3-millisecond std-dev stratum 0 time source. BIOS is open source CoreBoot and schematics are available freely. Used to be orderable for years, now with chip shortage remains to be seen.
It's what I compare all those chinese fanless router boxes to. Not one has come close.