digitally record some unpredictable source (or sources) long enough to get more than 256 bits of entropy, hash that record with a 256-bit hash (SHA-256, BLAKE2s…), and voila, you have your master seed

SHA-256 isn't a good "extractor" for all sources? SHA3 might be.

For a more formal treatment, there's constructions around randomness extraction and generation, like HKDF & NIST SP 800-90A (DBRGs).