How to secure an EMMC
If the attacker has physical access to your device, you've basically already lost. A state-level attacker can do whatever they want. A well-off corporation won’t be too far behind. Even a well-equipped hobbyist with basic tools at home can do a lot.

If you have a vulnerability in which physical access to the path between your storage device and processor compromises your design, I argue that you need to try and change to a device and protocol that transmits encrypted data over the wire. I'd assume storing encrypted images on eMMC would be decrypted in memory on the processor, so perhaps capturing SD traffic would not be the biggest deal, but your decryption key has to be stored somewhere secure.