Is truncating a hashed private key with SHA-1 safe to use as the symmetric key for AES for data at rest?
I see no reason why that wouldn't be secure.
If you want to be sure, though, you could use a standard key derivation function such as HKDF or PBKDF2 to derive the symmetric key from the RSA key.

These functions are primarily designed for deriving encryption keys from low entropy-per-byte sources such as passphrases, but there's no reason why you couldn't use an RSA key as the "passphrase".  In any case, what they're designed to accomplish is to distill the entropy from a long but not entirely random input down into a pseudorandom output suitable for use e.g. as a symmetric encryption key.

(Many KDFs also include features such as adjustable work factor parameters for key stretching, useful for slowing down brute force attacks on low entropy passphrases.  However, for your application, you may safely choose a KDF without such features, or just set the parameters for maximum performance.)