I've been working backwards from what I want to end up with. Taking the indie smuggling station idea I thought would be nice on the central main.
I wanted it to be a converted industrial station with shipbuilding capacity. The indies wouldn't be able to build such a station themselves, too expensive. But taking over a derelict facility.
At the time of the first human exploration of the sector, a gas giant was found with copious amounts of easily obtainable substance, call it G for now (will come up with a good name later). G is an imporatant catalyst in j-drive manufacture, and is rare in nature. Synthesising it was v expensive, so a natural source was major find. A small mining comglomerate got hold of it, and kept its existance secret. Then it began selling cheap G on the market and in a short time had virtual control of interstellar travel. With the wealth this company built a huge station to cope with production, and shipyards to build the ships to carry it. The Corpies found this intolerable. (The Sol Sphere would have technically imperial at this time, yes?, but I'm unsure as to how much actual influence it would have). After some time (50-100yrs), a new process is developed (or perhaps leaked by the hivers or the imperium) that makes synthesised G v cheap. This bankrupts the G company. Also some Corps get wind of the stations location and attack it, killing the crew and taking all the hi cost gear, leaving a gutted shell. Over time it gets used variously as pirate haven or seeker base, as the sector is slowly settled. Eventually it becomes a ragtag port, with repair facilities and building capacity.
Here's my timeline.
Discovery of G gas giant - 550
Early production starts - 580
Company stranglehold of G market - 600
Large station built - 600-620
synthetic G process - 700
attack on station - 710
pirate base - 780 and 830
belter base - 850-900
port type C 900-920
port type B 920-970
port type A 970+
There's probably a large belter pop in the system. I'd envisaged lots of belts and one rocky planet with some low grade Ancients ruins. The Civil War would give the company opportunity to get entrenched. the sector would have been mostly uncharted at this time, so the bulk tanker traffic, probably running silent, might give rise to 'black ships' myths.