For automatically unlock encrypted drives I followed the approach described in …stapelberg.ch/…/2023-10-25-my-all-flash-zfs-netw…

The password is split half in the server itself and half in a file on the web. During boot the server retrieves the second half via http, concatenates the two halves and use the result to unlock the drive. In this way I can always remove the online key and block the automatic decryption.

Another approach that I’ve considered was to store the decryption keys on a USB drive connected with a long extension cable. The idea is that if someone will steal your server likely won’t bother to get the cables too.

TPM is a different beast I didn’t study yet, but my understand is that it protects you in case someone steals your drives or tries to read them from another computer. But as long as they are on your server it will always decrypt them automatically. Therefore you delegate the safety of your data to all the software that starts on boot: your photos may still be fully encrypted at rest so a thief cannot get them out from the disk directly, but if you have an open smb share they can just boot your stolen server and get them out from there