Nice explanation. However you are describing a scenario where you have 256- bit key and you have to loop through all possible keys and try to decrypt the ciphertext with it - brute force attack. Yes, in such scenario 256-bits is plenty good enough.
There are other attacks that you do not cover. Such as the key was not selected in random or there is non-brute force attack. Or symmetric cryptography is not sufficient in the given scenario and you need to use public key cryptography where the key has different properties