As a general rule, I expect a file on an encrypted disk to be unreadable for anyone who lacks the encryption key(s).
> if I need to obtain some other thing from the original computer to "unlock" that file, I should be able to do it.
You can export your passwords to a plain text CSV from the original computer.
BitLocker does the same with key protectors. Even on a local account with no internet, there's always a 48-digit recovery password that opens the volume on a completely different machine.
Two of the three major platforms ship a portable passphrase path alongside the hardware one, so this being unavailable here is a product decision, not a law of key derivation.
Your average non-technical user will absolutely expect it to work this way, and if it doesn't, will be at best confused, and at worst incredibly angry that their photos are gone.
Exporting a private key from a keychain is hard to automate (the password of the keychain is asked for all exports, and HAS to be typed manually IIRC).