full disk encryption only protects you from passive snooping. If someone has physical access between two of your subsequent uses, no amount of any type of encryption will save you. Except maybe some entangled quantum bit collapsing mechanism. Maybe.

Think hardware keyloggers, fake MBRs, &c.

OP's trick won't work, but that's an "implementation detail;" there are plenty others that will.

EDIT: to clarify; that's not what you said, it's just a common enough misconception that it's worth being explicit about, here.

Of course keyloggers etc. are a problem. But that is a different story. A bug which can be exploited just by grabbing any device might have a larger impact on the vendors reputation. A keylogger, fake smc, whatsoever ist much more dangerous for a single person, because the attacker knows what he wants on the specific device.

There's have to be a hardware mod if a firmware password is enabled. It prevents booting from any other media or partition without a password.