So, after over 7PB written the 850 Pro has finally given up the ghost. It's been an amazing run over the past 9 months, and the drive has vastly outlasted its puny 150TB warranty - by a factor of more than 46 times.
Interestingly, the disk didn't fail while writing but during other testing. When attempting to enable drive security, an error code was returned and the SSD stopped responding to commands. And despite numerous attempts to revive it, it is completely bricked.
In conclusion then, this is an amazing device - fast, consistent, long-lived - overall excellent!