symmetric crypto typically wants the eat the cake and have it too, it wants to be both secure and efficient.

to that end, a so-called "security margin" is guessed at and the number of rounds of the cipher is determined accordingly.

it is certainly possible for an LLM to prove that the guess was wrong and everything that it implies.

having said that, the security of symmetric cryptography relies on the fact that you cannot unwind (find initial conditions) a sufficiently chaotic system in the discrete domain. for example, SHA256 with 512 rounds will almost certainly count as sufficiently chaotic by any definition but it wouldn't be as efficient as the current 64 rounds.

it is often said that it's difficult to come up with a secure symmetric cipher on your own, but assuming you know what you are doing it's quite easy. the hard part is to have enough confidence in it to make it efficient.