Standardized interoperable E2EE This isn't the thing you want it to be. Somebody has to broker connections and/or key exchange. You don't get to arbitrarily

Standardized interoperable E2EE This isn't the thing you want it to be. Somebody has to broker connections and/or key exchange. You don't get to arbitrarily send messages to the device of somebody on the other side of the world unless a 3rd party is providing those services.