How secure are conversations in circles?

My bishopric is starting to use the bishopric circle for communicating with each other, but there has been a concern about sharing confidential information in the app. Can we as a bishopric feel confident that any confidential communication within the bishopric circle is secure and will not be leaked / hacked?

One solution we suggested for ourselves is to delete any messages with confidential information after a week or so. How does deleting messages work in the backend? Is a message permanently deleted from the database when we delete a message?

Another side-aspect regarding security, is that (from what I understand) circle conversations like this are directly linked to the member's calling. So when somebody is released they won't have access to these conversations anymore, which is likely a good thing if you're concerned about confidential info. And a newly-called Bishopric member would immediately start having access to these confidential circle conversations, as opposed to not seeing anything in previous email or text conversations.