Elixir reuse channel functionalities when only connected to 1 channel

Question

I have a lot of channels doing different things like the following:

starting_channel
game_channel
food_channel
chat_channel

And there's maybe 100 of them.

Everyone starts by joining starting_channel. Later I can subscribe the socket to other channel topics and broadcast to different topics accordingly. But the thing is I would not be able to reuse the handle_in functionalities in those channels because users don't actually join them in the first place.

For example, let's say room_channel has this handler:

def handle_in("show_room_happy_users", _payload, socket) do
  broadcast! socket, "happy_users", Presence.list(socket)
  {:noreply, socket}
end

But since you didn't join this channel when you send show_room_happy_users message through the starting_channel nothing would happen.

Is there a way to achieve this without joining all the channel at once?