Sounds reasonable, but the secure boot mechanism of the Pi not only allows verifying the boot chain but also enables you to implement disk encryption with keys

Sounds reasonable, but the secure boot mechanism of the Pi not only allows verifying the boot chain but also enables you to implement disk encryption with keys stored in the the hardware itself that you can then only access from the running OS. Stealing the Pi or just taking out the SD card will not allow access to the non-OS parts. I'm not sure if the secure boot stuff of the Pi has ever been thoroughly verified or exposed to serious attacks, but in theory that's all possible.