Hello @MaxiMuss ,
welcome to this moderated Azure community forum.
You want a messaging solution where C++ clients listen to a central service without any individual identification.
This rules out IoT Hub and probably also the recent EventGrid MQTT Broker.
You should take a look at either SignalR services or Azure Web PubSub.
This is because these are designed for 'anonymous' web browsers using a generic protocol.
Both are basically wrappers around HTML5 Websockets (although SignalR is a bit richer with the capability to switch back to long polling etc. when the client or connection needs this). The scaling of the backend is arranged by Azure.
Check the FAQ for more details when you need to choose.
Clients still need to know how to connect to the service in a secure way (with some credentials) but there is no individual client registration needed.
The requirements you wrote can most likely all be implemented using on of these services.
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