While encrypted psuedo random is a more complex function, it is still PSUEDO random not real random. Future output is a function (now a more complex function) of past output.

Not only the secret key must be kept secret, but the seed (either current or original) must be kept secret.

If the attacker knows the algorithm, which they might, then they may be able to analyze the output. If they know the seed, then the problem becomes one of a known-plaintext attack upon the encryption algorithm. If they know the encryption key, it becomes a matter of finding a seed that generates some known pre-encryption output of the psuedo random algorithm.

We could go back and forth about the practicality and difficulty of such attacks. But switching from psuedo to true random eliminates that entire discussion.

Your $10 webcam + lens cap is a good idea BTW. Or other similar ideas of using various sensors to capture random noise. Just hope the noise really is random. Also, a $10 webcam without a lens cap, pointed at something, like a busy street, might also be a source of randomness in the pixel data.