Hacking a signal server should yield zero useful results. Messages are encrypted on the phone before being sent. Signal servers only ever receive and retransmit encrypted blobs, never the plaintext. They, by design, do not have the keys to decrypt those messages. There might be metadata about who messages who and when, but I’m not 100% familiar with that part of it.

Now, if you pwn the phone, on the other hand, you can record the display and log the keystrokes.