> Also of note, is Ed25519 does not harden itself with additional "bits" in the normal RSA sense

Yes, it does, it is just not Ed25519 anymore, check out Curve448 and EdDSA-Ed448, same Edwards curve construction, more bits, more security. It's just that Ed25519 is plenty strong for many use cases.

Also, don't confuse the strength of the KDF with the strength of the curve itself as others have noted - Ed25519 is the same strength whether you hash your passphrase 1 or 50000 times. But with the 1 round your key might be easier to brute force if you have a weak passphrase.