So I just had a game with a huge duranium crunch, and I was rescued by the civilian economy, so some tips:
The shipping lines seem to require that you have a nuclear thermal commercial engine developed, as well as a colony with infrastructure on it before they build a colony ship. And they build more if the ships they have are in use.
So once you see them build a freighter, make sure you have contracts for it.
But be sure to take breaks in those contracts so those freighters will ship you wealth and infrastructure on their own.
It seems to take about 2-3,000 infrastructure on Luna (with pop to match) before the computer really starts shipping your civilian produced infrastructure. As I had a major duranium crunch coming up, getting the civilians starting producing infrastructure was essential to my plans.
I used a small colony ship with LOTS of cargo handling to ship colonists back to Earth, so that Luna would not become overpopulated, and worse, so the civilian cargo ships would not go idle, as I needed the wealth flowing to get them to build more and more ships.
One of the quirks of computer colony ship and freighter design is their lack of cargo handling systems. So I would really like to know if their loading times are affected by Spaceports. If so, you can greatly increase the return on the Earth Luna runs, and to a lesser extent the Earth-Mars runs by building Spaceports. It is fairly simple to build a Spaceport on the moon, even though you can't ship them direct. Just build a small freighter with a lot of cargo handling systems and shuttle factories to the moon, + the materials for it, and then ship the construction factories back when done. Two spaceports on earth and two on the moon and you almost triple the income you will get from shipping colonists.
And once the moon is over 25 million, you can make money periodically reversing whether the Earth is the source or destination of the colonists.
16 years in to a slightly boosted conventional start (800 million pop, 9 research facilities, 3 naval academies) I have 29 civilian mining facilities. Earth has been 6 years from running out of minerals for a couple of years now, as I frantically convert mines to automated and ship them out with the civilians. I wouldn't have been able to afford duranium for the ships needed to ship all my mines out and build the mines. It was touch and go as duranium fell below 2,000 a couple of times, and i had to shut down shipyard expansion and shift production to converting mines, but now my economy is back on track, all thanks to my early efforts to boost the civilian economy. +20% research boost to the civilian economy and +wealth leaders and the Sector Command bonus helped too.