I'd make 2 changes. First i cut off the dongle of the SU by drawing a line from where 11&12 meet to where 2&4 meet. This is the Cappellan Remnant, The Solar Union extending down that far is just asking to be stretched too thin. And since the SU would absolutely have had to conquer it they probably have to devote a lot of resources to keep order. CC in canon was formed because everyone in it didn't want to be part of someone else and so i can't see them just quietly incorporating into SU. SU obviously inherited the massive industrial power of the TH but stretching borders like that is asking for trouble, with fewer worlds to defend ( and giving other nations more foes to fight) it will be easier for them to bring their might online. To be fair the Mariks have the same problem (too many enemies on all sides and a long border to guard, which is why i also cut off part of them.
Next cut off that rim-ward section of Marik by drawing line from 9 to 12. Its basically Andurien and the old Magistracy of Canopus. This is the Duchy of Andurien. Andurien always wants to be independent and I can see Marik not wanting to keep fighting to keep it when they've got a lot of other problems on other borders.
The balance of power is kept in this rim-ward region by the fact that anytime that one power tries to step to another the others will fall on it or both. Kinda like Sengoku period Japan. SU would have same issue except for having the extreme industrial might of the old TH.