Whenever you have got alot of planets under your control, the taxes start coming in in small amounts, and the max you can alliegance u can get in a different star system w/ culture is 40% (tec) 35(vasari) and i dont know about advent. But i think they should make a new system or stucture, maybe a town hall, or maybe a planet upgrade that makes alliegenace higher. In every star system you should be able to make like 1 town hall which will make your alliegance of your planets in that star system slowly climb up to 60 or 70 and the planet which the townhall is in would have an alliegance of 80%. also maybe population infrastruture could also raise alliegance if the town hall wont work. These are jsut ideas so please throw yours in too. (i know almost noone plays multiple star systems, but what if u have 2 terrans on dif sides of the map, thats messed up that u get almost no tax from it.