Since it seems that noone has ever answered this question here or in the Enterprise forums, the half baked Sandisk Guard Tool utility doesn’t work with its default firmware update settings under Windows.
The method to get it to process without errors is to use the non-default firmware update mode (second option in the list I think) which allows you to specify the offset / block size. Via trial and error with common block sizes, I found that a block size of 256 was accepted and updated my Ultra Gen II drive from ZR07 to ZR08.
Even with the latest update on my drive, performance is still very marginal on consumer-type controller platforms (Intel SATA3 in AHCI mode during my tests) and light-load / QD1 performance is conspicuously low. This leads to very bad scores on consumer-type benchmarks such as PCMark tests, so I wouldn’t recommend people snap these up to use for non-server environments. Their firmware is likely specifically optimized around LSI-based hardware RAID controllers vs consumer controllers, so I wouldn’t expect improvement this far into the firmware’s development either.
There seem to be plenty of bricked versions of these drives floating around eBay with customized non-generic firmware, so updated firmware appears to be crucial for long term reliability–and that generally isn’t possible if the drive is not a generic one that can be updated with this unfriendly / DIY-class-support enterprise utility.
