GrapheneOS is limited to specific hardware. And a strip of thermite will absolutely end any flash memory, while being stable and safe to handle. 300c isn’t that hard to reach.

Legally the problem becomes the existence of data. If it exists, it can be subpoenaed, and you can rot in jail until you comply. Destruction has a simpler hurdle of making sure it happens due to others negligence or before it’s under other jurisdiction.