No change with write cache buffer flushing turned off. Power is on high performance. Indexing is turned off.
Drive has 199GB free of 466GB. Installing Magician confirmed the drive is using both the Samsung driver and PCIe Gen 3 x4.
Not sure if using the PCIe add on card would make a difference, seen some commentary on that solving similar issues, but that's very disappointing if that's the solution.
One thing, turning SATA Mode in BIOS to M2 instead of SATA Express does not seem to be retained. The board chirps, and then if I go straight back into BIOS the settings is back on SATA Express.