Another factor to consider is the level of automation on these ships. In today's world, there is already a lot of research going into self-driving vehicles, with an eye towards replacing truck drivers and the like.

Is anything more than a bare bones crew required? Are there roles that need to be filled by humans that cannot be automated? Maybe a majority of the people on these ships are on a one way trip.

The ones who choose to travel back and forth, watching the rest of the world move at 12x speed, could be seen as very odd by the rest of society. Each individual might have their own reasons for doing so. A person who is down on his luck may want to jump ahead a few decades to find a new start. Maybe a curious sociologist is researching the long-term development of culture on the colony, etc.

I would not want to overcomplicate your original concept, but the idea of communication devices using quantum entanglement also popped into my head. That is, if access were extremely limited and jealously guarded due to low bandwidth and high monetary cost of the device itself. Possibly another interesting social factor.

I'm sceptical about trade if the return of investment will be seen in 24 years, it's too long for corporations since the CEO that arranged it will be fired/retire in the meantime Maybe asian corporatrions could do it, but western ones, no way I think it would be more into a form of transports than trade. I mean, since the schedule is so terribly important in this case and because it requires such high political stability it would need to be about something else than simple monetary gain. Also I suspect a planned economy system.

The route probably yearly (a new ship sent each year), cargo simple and strictly standarized and predictable (you have to change the cargo 12 years in advance).

Probably emergence of spacing guilds. Since it takes 1 year relative time to travel, the simpliest solution is to form a new class of citizens that live in a relative time and form a separate society (they all are from the "old era", after like 2 trips).

For passengers return of the investment will be just in one year. It would be high risk gambling but could be very profitable. If group of people have ship and some valuable good that they think is still worth of the money after 12 years in another star system then it's not that far fetched idea.

(I feel like I have read science fiction along these lines in the past. Bonus points if anyone can point me to it)

My bet would be A Deepness in the Sky.

The timescale would be a lot longer, but making a Qeng-Ho-style game opens up a lot of neat strategic possibilities, like seeding a civilization with the technology at visit 1 that will allow them to engage in higher-value trades at visit 2. But as your trade with them and they get more advanced, so does the possibility they wipe out their civilization (either back to the stone age, eliminate their entire culture, or wipe their planet of life entirely).