Very interesting post. But i feel the need to clarify that it's actually not actually possible to definitively prove that the universe is superflat
Keep in mind that no matter how much distance you use to measure the flatness of the universe, we can ONLY speculate what the results mean for the observable universe. The issue is that the universe is larger than the observable universe. The background radiation we can perceive only defines the observable universe, as we know that the universe is expanding faster than the speed of light, we know that there is more universe beyond said radiation that we cannot observe.
Imagine you are given a section of a line that you have to determine whether or not if it is curved.
Now say that it appears to be flat. How do you know if that your apparently flat section of the line is not just a tiny section from a much much bigger circle where the relative curvature of the section appears to be negligible?
That's my 2 cents.
Of course we can't conclude anything yet, especially when there is so much with unknown properties out there, there is no way we can say that the observations we've made can apply beyond what we can observe.