In my experience, stopping to talk even for a moment already makes it submit. This makes a real conversation with pauses for thought difficult, because of the need to hurry before it cuts off.
I think you’re describing the conversation mode (started via the headphones icon), I also have issues using it. But you can also dictate a message, on iOS it’s the little gray wave icon on the right of the text input. With this mode there is no auto submission.
In my experience, stopping to talk even for a moment already makes it submit. This makes a real conversation with pauses for thought difficult, because of the need to hurry before it cuts off.