You could just use onion services to to hide the server, and store some backup onion services (whose private keys are kept offline) within the application or its files. When the server goes down due to seizure, you spin up a new one under the backup service's pubkey, and sign a list of new backup keys which will also be kept offline until the next seizure.

You could also combine encryption with steganography, if you strip non-random 'protocol information' from your encrypted bits. Doing that, it would not be easy to prove that you are sending encrypted messages at all without having obtained your keys.