Feeding people on a massive scale doesn't require industrialized farming, in fact you get more calories/nutrients per acre if you use the standard pre-industrial farming model of mixed animals and vegetables.
In fact, if this society is so backwards, they would probably HAVE to go over to pre-industrial farming techniques, since modern industrial farming depends upon modern science and medicine to keep the entire system from becoming a plague-filled pile of rotting cattle. I imagine that when humanity was first settling planets, they had to have dedicated non-industrial farmers; the reason is that even on Earth, whose biology we're used to, it's really hard to keep up with keeping our cattle from keeling over or our corn from getting killed by a single enterprising virus or bacteria. If you go to another planet, whose biology you're not used to, and you try to monoculture farm... well, the planet's ecology will probably just laugh at you and watch your disease-riddled crops rot to dust. If this planet got to the colonization stage, but never received the advanced science or technology required to advance it to the industrialized-farming stage... they might not even have a choice, never having developed the antibiotics or genetic redesigns of local plants required to turn it into an industrial world.
The rule for pre-industrial is that for every 10 people, 9 people need to be farmers in order to feed the other 1 person, who will be off doing sciency or administrative things; considering that you've described a planet that has a mediocre industrial sector and technological sector, AND doesn't have enough food surplus to warrant major exports, that model seems to fit your planet's statistics nicely.
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I don't know, I'm just spewing random things I know and speculate on off the top of my head, trying to inspire you with random thoughts. There are a lot of reasons for the notion that this planet simply is not advanced enough to support mass farming in the way Earth today is experiencing it. But that's only one idea, pick the one you like best =P
On another note, I'll quickly relate a planet in MY game that is actually running on similar terms to yours... maybe you'll get a couple ideas. Or maybe I'm just showing off a cool planet I made, who knows, right?
74 Orionis
Noble Ruler: Chairman Yu Xióng
Star Type (Recharge Time): M1VII (202 hours)
Position in System: 2nd of 6
Time to Jump Point: 2.96 days
Number of Satellites: None
Surface Gravity: 1.19
Atm. Pressure: Standard (Breathable)
Equatorial Temperature: 27 °C (Arctic)
Surface Water: 67 percent
Recharge Station: None
HPG Class Type: None
Highest Native Life: Fish
Population: 32,400,000
Socio-Industrial Levels: C-C-A-C-F
Its history is that it was a terraforming project located near Terra, initiated just before the Amaris Coup. When the Exodus happened, the SLDF folks on the planet left and took their dropships and jumpships with them, waving goodbye to the colonists and saying "We'll totally tell people you're stranded here, I'm sure they'll pick you up." Predictably, no such rescue ever happened, and the icy, asteroid-laden mining colony was left to its own devices on a planet which wasn't even advanced enough to have plant life, just scum on top of the ice-cold water that the fish would suck up. At least they had a half-finished biodome, a single fusion reactor, and equipment for strip mining and terraforming the planet. They dug into the ground where the atmosphere was thicker and warmth could be retained, and they did the best they could to maintain the terraforming equipment, mine raw materials, and build a society while they waited (and, eventually, gave up waiting).
They eventually got, by dedicating their entire population to the task, a Class C Agricultural Output rating composed of mostly native fish, pond scum, and imported soy beans in the biodome they struggled to finish. But then, near the year 3,000, pirates jumped into the system to evade capture by Lyran forces. They realized that, though merchant work wasn't their thing, if they would just keep the planet hush and supply the locals with food, they could force the locals to create a gold mine of raw materials. Though living under the control of a company of pirates wasn't exactly ideal, the planet was able to get its Industrial and Technological levels up at the cost of its Agricultural level thanks to the steady supply of food.
The mercenaries the players are playing have been hired to bust into this secret pirate star system, steal any and all Star League era terraforming technology, and get out. As it turns out, the corporation which hired them found out about the planet when another corporation found out and kicked the pirates off, and the pirates, being irate that their retirement plan was stolen from them, told the second corporation and plans to attack at the same moment the second corporation sends the mercenaries, hoping to kick EVERYONE off planet (except for the colonists of course) so they can have their cash cow back.
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There's something about the idea of a technologically backward planet that is struggling agriculturally and is being protected/abused by pirates which is very appealing to me =P