First things first, there is no app like you describe. You have to take my word for it. Let’s say a certain country’s law enforcement might be very interested in taking to me if it knew role in certain events.
I examined many messaging and chat platforms, and the closest to what you’re asking for are custom Telegram clients that go a longer way to ensure that TG’s local data gets deleted properly (by default it doesn’t, easy peasy to get data from the local DB, half cleansed chats from 6 months ago pop up in exports or even the UI, etc).”, has a built in double bottom for accounts, etc. An example of what I am talking about is here: https://github.com/wrwrabbit (check the repos yourself).
Secondly, you must understand that when the law enforcement seizes a phone, upon initial quick examination (sometimes using brutality to force you to unlock it), they will shut it down to ensure it is not connected to the Internet. And then criminalists will use a special black box device sold by an Israeli cybersec firm (its name always escapes me, but I think the product might be called Pegasus) to extract all data for offline analysis.
So thirdly, you must not rely on any app that conditionally removes data from a device. You must not entrust your well-being to it. Which leaves you basically the only choice: only ephemeral messaging capabilities which are available only when you are in the app, through the server over which you have full control.
Or your messaging sessions must take place only when you’re in a secure location. And even then you should have a “wipe all button” in case the police come after you unexpectedly.
Man just settle down that what you are asking for is not available on the market off the shelf.
It’s probably not an open discussion to talk about your actual scenarios.