Sorry to revive this super old thread, but as I understand it, there will still be metadata left on the device that would be readable, such as directory structures, number of files in each directory, approximate file name sizes, approximate or exact file sizes. If the files are downloaded from the web, or have been uploaded to the web, file confirmation attacks would still be not just feasible but rather trivial to do. The rationale that wiping a key from a secure element would render all data unreadable is really only true in the case where 1) the device is random initialized, and 2) encryption operate on block device level rather than file system level. Most SSD disks for computers do transparent encryption, and would securely wipe the encryption key and generate a new one if using ATA Secure Erase or NVME equivalent, at least if they implement the specification correctly.
So, I think the question remains, is there a way to reliably wipe the actual storage memory, as in forcing erasure of the memory blocks (not just TRIM/DISCARD'ing them), or forcing the rotation of a key operating on block device level like on SSD disks? Since GrapheneOS does filesystem based encryption I think this is even more important that it would be for a computer running block based encryption like LUKS2 or VeraCrypt.