TURN server doesn’t need to be free, you just need everyone to be able to access it.

The product I work on in my 9to5 would be perfect for your use case from the technical side of things but sadly the commercial side is a completely different story that makes it not even worth recommending.

Rocket.Chat seems to be doing well in Europe as well; they recently released their own native matrix implementation.

Any Meta app in your phone can read it whenever it wants. It just doesn’t send the message itself to their server.