On this at least we can agree. You could use both, but if the in game one gives you a different runway, for example, and you follow the external ATC directions, it won’t trigger the Objectives stages correctly, leading to a takeoff not getting logged, if they don’t match. If they did match, it woul work as the in-game one doesn’t care which route you take when you hold short, only that you get to that spot. You could follow the external ATC taxiway route to the same runway.
It would be easie if they came up with a more basic method for determining a take off, such as wheels leaving the ground. 