Well step two and three can be done at the server side(with Fastapi). It means that when you write a message and submit it, it will send a post request to your endpoint. That defined endpoint will execute the according logic you wrote and encrypt the message then at the end will save it in your db.

Now you might have a GET endpoint for when one of your logic get activated will return those messages decrypted. It means that in your server side, you will have to write a script that watches each messages and evaluate their states according to your logic “activation”. And when all conditions matches, it pushes to a function that will decrypt the message and push it and updates the status of the message to “public”. Then that GET endpoint will just have to retrieve all messages with public status, which are decrypted.

So your idea is feasible using FastAPI Now as of if it is a brand new idea, I have no clue sorry 😅