Draw the occluders twice - first time to determinate fully covered texels and remove all list entries on their back.
Conservative rasterization would help here too if it's possible to draw only fully covered pixels.
I think normal depth only pre pass without conservative rasterization would fix most of the overdraw problems. Second pass would use lequal test. Then additional optimization could be possible to skip all point in frustum test for Equal depth test pixels but I don't see that test as bottleneck.
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.