I understand the point that you are trying to make, but I do not see it as a fundamental flaw within the Federal system of governance that you are making it out to be (making an assumption there, feel free to correct if it is wrong).
You are correct, and if a citizen does move over to another area of space in a different District, they will be represented by someone different than their original senator. However, I wouldn’t posit that is necessarily a drawback to the Federal system. The sub-district would elect a Senator that is local to them, and therefore has a greater understanding of their needs and requirements when representing them in the Senate.
In respect to my earlier point that member states can be tailor-made to the cultural requirements of their inhabitants as long as they comply with the terms set out in the Federal Charter and other instruments of Federal law. The member states are what the average person in the Federation will be interacting with on a daily basis, not the Federal Senate.
Additionally, organisations exist to promote and safeguard the welfare of those that reside in the Federation which are not bound to territorial jurisdictions only. My own identity as a Caldari is represented across the Federation by pan-Federal groups, as are the Matari, Mannar et al.
It might not be a perfect system, the Gallente would be the first to admit that. Before I got a grasp and witnessed it in operation myself I held nothing but disdain for it. But it is ever-changing and driven by the desire for self-improvement which as a Caldari, I have nothing but respect for.