Just my take.
On a world feeding 10 billion plus, unless there are fertile worlds in system, food is going to be a constant issue. Add in foods common in other systems, but not native and there is a huge market.
While there are technologies to supplement the need for open space, agriculture is very land intensive.
Add elements and compounds that by definition are more common on some worlds, and less common on others, and I have always though the need for commerce is grossly understaed by most players.
I see a mufti tier system. Along the major trade routes are huge haulers that have absolutely the bare minimum drives, comps fuel and personal requirements to move freight along to the next system in the loop. just as with the large container ships we have, they port JUST long enough to load unload and fuel, and jump again. The crew spends all of the jump doing the routine maintenance so that downtime for annual maintenance is kept to a minimum.
These haulers are outside the 100d jump limit, so that no fuel is carried or time is wasted moving into the jump shadow and back out.
That is where the 1000 credits per dt applies.
Those routes rely on highly standardized, cargo containers, and cargo that is the same loads month in and month out. The ship are very standardized, with bare minimum equipment and there fore repetitively cheep to build and maintain, and kept in service as absolutely long as they are space worthy. Being so far from the planets they serve, meand that anyone not actively involved in their operation probably never sees them personally, although they would certainly make good backdrops in vid and promotional advertizing.
They are also exclusively owned and operated by the Mega corps. They are served by a huge fleet of lashes, tugs, and insystem craft flowing to and from high-ports situated above the major ground terminals of each populated world in the system. They should account for maybe 60 to 70% of the cargo in transit along the main trade routes.
NO regional hauler has any access whatsoever to this traffic, although some of the in system transport MIGHT be contracted out.
As far as subbies and independents are concerned, this is cargo that might as well not exist, becuase they will NEVER see a piece of it.
Off the routes big enough to support the hyper haulers, the regional carriers would have a smaller network of smaller ships to haul to the feeder worlds.
Of the remaining traffic along the mains, most of it would be regional haulers carrying loads that are not consistent month in and month out. Priorities, overstocks, specialty orders, etc.
Again the bulk of that cargo will be standing contracts at the given imperial standard rate.
The small fraction left, (still significant to the smaller freight lines, and the independents, are the one off cargo, there will be smaller producers that can't or won't contract the Megas and the feeder lines. There will be small runs that are not viable for steady contracts, special orders, small priority contracts, and loads that just can't be routed through the normal process.
None of this cargo is handled through the megas, and I have always believed are NOT subject to to the Imperial subsidized common carrier fee.
The Imperiam guarantees that you can ship for a set price through the Megas and the feeders for that standard rate. shipping outside that system is NOT common carriage and prices are negotiated.
This means that a pc craft will virtually never even have access to a standard price cargo, and must negotiate every single cargo. Most of these cargos would be speculative, but there should be enough small cargos that are low priority that are not going to pay much above standard rate that there should never be a ship in transit that can not fill their cargo space except on the tiniest words.
The same applies to passengers. There are huge terminals run by the Megas and the feeders, that run passengers like cattle for the published prices of passage. Those passengers are mostly going to fly the established route. You are not going to book a trip, and by a formal passage unless you already know you have transport booked. You are NOT going to wander around with a ticket and then go into the slums looking for somebody that will honor it.
So. The pcs will have opportunities, but they have to scrabble to recruit them, and price will ALWAYS be a matter of negotiation, and haulers will often have to accept loads at or below the Imperial rate, although rates above the Imperial are also possible.
The Megas make money in volume. Their overhead per ton is FAR below what anyone else can mach, and they are the only ones that can make money at the Imperial rate.
Subbies are subsidized by the worlds they serve to make money at that same Imperial rate.
The independents are on their own, and while the savey ones can make the connections to make money, the less skilled, lucky and connected lose their shirts.
The net result is you have a big ship universe, but PCs NEVER interact with it, or even see it. It is window dressing, although merchie characters that served the Meags before they retired would have been in that system.
The other fear people have with a big ship universe is teh navy.
I don't care what size ships you deal with, a mercie that has a direct encounter with a capital ship is a dead merchi. You will NEVER encounter one under normal circumstances, and if you meet up with one at the business end your chances of living to telll the story are virtually non-existent.
The places a pc is going to interact with large ships, are as passengers on large liners, or tucked into cargo on the large haulers. Again, hijacking those cargos would vertually never be profitable, and hijacking the ship would never work. If you had the resources to process such a ship in any way, you would have the resources a mega corp behind you, and those resources would not be idle waiting for the opportunity to hijack the ship.
By the same token, those ships would never leave the mains, and so never be in a system that was not well defended against piracy. There is not transit across the jump shadow and no high velocity maneuver more than a few light seconds from help.
I could even be comfortable with a main drive that is less than 1 gee. There is no need to move any further than the jump shadow of the terminal, and even at sub G acceleration the transit time would be negligible. Less drive equals less cost in drives, and maintenance, and fuel, increasing capacity even more.
Lots of detail, but then I have been considering these issues for many years,.
