If you’re worried enough to encrypt the drive, you shouldn’t be auto mounting it.
This really depends on your threat model. If you are only concerned about the drive getting stolen, or wanting to keep the data on it private if you need to RMA the drive, mounting it automatically on boot with a key stored on the rootfs can be perfectly fine. If you are a journalist in a hostile country and protecting your sources from state level actors is a matter of life and death, then yeah, this would be woefully insufficient.
This really depends on your threat model. If you are only concerned about the drive getting stolen, or wanting to keep the data on it private if you need to RMA the drive, mounting it automatically on boot with a key stored on the rootfs can be perfectly fine. If you are a journalist in a hostile country and protecting your sources from state level actors is a matter of life and death, then yeah, this would be woefully insufficient.