Hi, interesting ideas! There is actually already a mechanism for wealth distributing to neighbouring stars, so you gain some wealth from richer neighbours and lose some from poorer ones. But it isn't tweaked for galaxy size, which is probably fine until you get up to rather big galaxies. I may tweak around with this for a while if I have time.
Right now, trade and distribution of wealth is largely apolitical, opportunistic traders will happily send their ship through dictatorships, cults or ALL WILL BE ASHES crews, moving between borders freely.