Generating 1024 bits of entropy from a seed I need to generate 1024 pseudo-random bits from a 100 bytes string seed (only valid lowercase chars)

One thing I'm pretty sure would be secure enough would be to repeat this 100 bytes string 20 times and encrypt it (using itself as the key with AES for instance) and then return the first 1024 bits

Does that seems fine?
Is there a more "standard" way to d that (some simple/secure key expansion algorithm)?
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.