I have the very risky idea about the problem with jumpholes, that would went into inactivity and be destabilised (like the jumphole in Tohoku, was it that jumphole), what would justify shrinking Sirius a bit. About the guard systems, we would re-use them and put into systems where are jumpholes to, akin to the idea of Zone 21 in New York.
I would combine Alaska and Ellesmere, because half of Alaska is just void, so there shouldn't be any problem to put second sun and fill half or 1/3 of the map with Ellesmere Badland-like voidness.
I am suprised a bit to see that we agree at some points... What about my idea of merging some systems (my idea of Pocket Sirius) in point 3: Omegas, Omicrons, Taus) into sectors? Or I should repeat that jumphole instability what would lead to cut off some routes.
Also... wasn't there one star (can't remind the system though) is very close to the supernova? I can recall it was one of the Gallic systems. If not, why Nomads are not able to blow up systems like they did in past with Sol?