encrypted by both a boot password and a secure enclave held key
Are you suggesting that both keys should be required for decryption?
How would you restore the backup if your secure hardware device is lost/stolen/damaged?
The backup would live someplace other than on the broken machine. It would be encrypted some other way (or maybe not at all, depending on threat profile).
No point in encrypting those family photos burned to disc.
And the owners of the computer can't get the key in the secure enclave? Are they part of the threat model?
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