Generating 1024 bits of entropy from a seed
You should use a well-vetted implementation of a well-researched key derivation function that can output at least 1024 bits.

HKDF-SHA-256 is the standard and secure  choice these days, although constructions based on SHA-3 and Blake2 are also in use.

If the characters you’re hashing are created by a human, use a specialized and intentionally slow password-based key derivation function like scrypt or Argon2 instead.