If your motherboard with updated BIOS and Phison PS5012-E12 controller are not making any problems than perhaps you might need to be a bit aggressive, do yourself some homework/workaround with your NVMe drive, if you want to resolve this on/with Linux (without blaming or expecting from PureOS maintainers to solve this), but as first please understand your standing point if you will, as already kindly described here:
Therefore, as you didn’t want to avoid proprietary or closed H/W, in your case probably key sentence is: “SSDs with overprovisioning space typically have used 7% of the total SSD storage capacity.” It is, for example, proofed that 960GB SSDs from manufacturer you are using occupy around 24GB with Pseudo-SLC-Cache. You might try to remove this space that Phison controller uses and look if able to boot PureOS from such/upgraded to 512GB NVMe drive (after new format). I’m not providing guaranty that this advice will help on Linux, instead it should be your homework (probably by using dd command within CLI, after backup). Again, my main point here is that you need to be active and not others. If you need any other, here related, help or direction just ask.
