security of using digital signature as key derivation material
I'm sorry, but chances are this doesn't work.

The reason is of course that ECDSA signatures are usually fully randomized, meaning that there's randomness introduced in between the private key and the final signature. If you're looking at an ECDSA specification, the relevant value usually is called $k$.

What you'd rather need would be an RSA encryption / decryption keypair or an ECDH keypair. In this case you'd just store an encrypted key as a file or something and let the secure enclave decrypt it at each startup and keep the key in RAM. The downside being of course if an attacker can extract it from memory this application's security is screwed. If you have a static ECDH key pair you'd just store a public ECDH share and statically derive the key each time and use it as suggested.

If you have to use signatures though, you need to find a deterministic one that is offered by the enclave and use it as proposed in your question.
This can include ECDSA if it follows any standard for deterministic ECDSA (like RFC 6979) and this can also include deterministic RSA signatures (e.g. usually the older signature standards).

Bottom line: As long as you use some (intermediate) value in the key derivation which directly relies on a hardware-backed key (in a deterministic way), chances are this will be secure.
An example of how not to do such a derivation is given in Android's FDE setup (with the counter-example for a good derivation would be iOS actually)