This is my understanding.
Although I think it's good to say one probably needs some level of samadhi before having insights, it seems to me you are still trying to answer this question on a level of intellect. But it's not like that. Seeing not-self and impermanence is not by intellectually excluding options or investigating all kinds of options. It's a result of seeing things directly
as being impermanent, not sustainable.
Tastes in the mouth don't last forever. Everybody knows this. You may eat something nice, but very soon all taste is gone or replaced by something else. So this taste was impermanent. You don't have to taste all foods in the world to see this, or have to understand how taste buds work. It's right there in your own experience. As a result, you don't expect any eternal happiness in food.
But it's a bit deeper because not a single taste can disappear, but the entire consciousness of taste. Often we are not aware of taste at all, there is no contact with this sense and thus no consciousness of it. So if this consciousness itself doesn't sustain, why expect anything eternal in it?
Now extrapolate this to the six senses. Do you get the idea? You don't have to investigate anywhere outside of the present moment. The problem, of course, is not so much taste consciousness. People can let this go quite easily. However, most people find it hard letting go of mind consciousness mainly. As a result, they have a sense of continuity in it and that's where a lot of people try to find a