Thinking of it, some said they would not like civilian harvesters to fly of to to colonies when full, so maybe there can be civilian fuel shipping lines that ship fuel instead?
Could even make the whole civilian trading and the universe come more alive. I remember in the previous aurora shipping lines bought new ships with money they earned. So how about something like this:
Have the fuel harvester civilian corporation that spawn fuel harvesters, they will just harvest fuel and do nothing when full. Oil rigs if you will. Then we have the fuel shipping lines, the oil tankers, they will fly around and buy fuel from civilian harvesters and then fly it back to the nearest colony. When they reach the nearest colony it either get sold to the civilian population which earns you tax income, or the player buys it automatically depending on settings. Or the oil tankers and rigs can be all be spawned within the same civilian line. Dunno what would work best.
Now how you set it is a bit of a question. Although, setting it on a per planet basis works well with minerals, so I guess you could say something like buy all sorium from this gas giant or sell to civs.
Now if you really wanna make it interesting, have it so friendly aliens and players can trade fuel, so they can set to buy fuel from aliens they have trade pacts with. And if you wanna make it really really interesting start simulating the price of sorium according to supply and demand. A harvester that is almost full will likely sell it of cheaper to get rid of it, while a planet with plenty of fuel won't pay as much, but a planet with low supply might pay high prices.