Sorting vast amounts of data is already an issue for intel agencies that theoretically llms could solve. However decrypting is magnitudes harder and more expensive. You can’t use llms to decide which data to keep for decrypting since… you don’t have language data for the llms to process. You will have to use tools working on metadata (sender and receiver, method used etc).
There’s also no reason for intelligence services to train AI on your decrypted messages, it won’t help them decrypt other messages faster, in fact it will take away resources from decryption.