Something like half of the comments are recommending using a password for the FDE key instead of the TPM. I thought that it wasn't normally recommended to use a hand-entered key directly, since they wouldn't be expected to have enough entropy for FDE. The better solution would be to store it on a TPM behind a password, where the password check happens inside the TPM, and it can destroy the FDE key after too many wrong attempts. And uh, not send the password unencrypted to the OS over an interface that can be tapped with electrical leads on the motherboard.