There's absolutely a secure system that crosses agencies - they even refer to it in the Signal chat (see the comment about sending details to your "high side in

There's absolutely a secure system that crosses agencies - they even refer to it in the Signal chat (see the comment about sending details to your "high side inboxes"). But you can't use that system on your personal phone, and it doesn't let you avoid record-keeping requirements by setting messages to auto-delete.