And this could also work via dropbear. There'd still be an exposed dropbear SSH key in /boot. But it'd be the TPM that you're unlocking. And I assume that there are ways to verify the authenticity of the TPM via initramfs, before providing a password.
Am I understanding that right? It does seem more complicated. Especially given that you don't have physical access to the server.
Yup. Nope. The key is protected by TPM magic :). TPM measures stuff, then we put the key in TPM and seal it to PCRs 0-13 measurements (whole boot environment). At boot, TPM will allow tcsd daemon (part of initrd) to read LUKS key once, and only if all measurements match, this is as far as I got.
End result is unlocked volume, without any passphrase prompts, neither on console nor via ssh.
If this thing works, there is no need for remote ssh FDE unlocking.