Not sure that "optimal" designs exist as it varies a lot.
For example, if you find a super luxury, I want a mining station with 4 Lux extractors.
Here's a copy of analysis I did on freighters in another post:
I think you misunderstood the intent of the comments or I did not explain them well.
Subwarp speed, or even the type of warp speed drive, is secondary in determining the overall trip speed. The number of resource pick up stops is the primary driver in determining overall trip speed.
Freighters have to fill up to a certain amount of cargo space utilization before returning to a spaceport to unload.
This isn't the case ALL the time, but most of the time.
Based on observation, most large freighters (computer designed) make 2.7 stops per run between spaceports picking up cargo. I increased the space by two cargo pods and this allows them to more efficiently clear out a three run mining station that has had a pickup in the middle of the economic cycle. I then tested this increase and it had no impact on the number of pick up runs.
Medium freighters, on average, make 1.25 stops. I increased the cargo holds to increase the effectiveness of that second stop. Same deal, a adding an additional hole increased the pick up stops to 1.35, which I didn't want to do.
Small frieghters usually make 1 stop. And usually leave cargo behind. So I increased the cargo size by 1 hold to make them more efficient. They still make one run and increasing the number of cargo holds to 5 increased the number of pick up runs to slightly over 1. Which I didn't want to do.
So large freighters deliver more cargo to its stopping point, but take longer to get there than medium or small freighers due to the number of pick up stops.
Medium freighters also take more time than small freighters but less time than large freighters, But they also deliever less cargo at the end stop.
Small freighters are by far the fastest due to making only one stop (at least about 95% of the time). But they deliever the smallest amout of resources.
Ideally, the game should put large freighers on long, high volume routes, medium freighter on shorter, high volume routes, and small frieghters on small volume routes. And standard routes should develop over time. Reality is, the game doesn't do this. Freighters go, usually, toward the nearest resources to be picked up. If that doesn't fill the cargo space, then they go to the next place. And so on.
There is some intelligence in the routing. If a mining station has a freighter assigned to pick up cargo (regardless of size OR the distance from the mining station), then the next freighter goes to the next mining station. And each freighter picks up cargo, this resets the priority and the game looks for the next place to send the freighter to top of the cargo. And, in general, routes get picked based on location to the freighter at the time it needs another pick up point. But if that happens to be on the other side of your empire.......
So the issue with large freighters is that they usually have to make multiple stops before going to a spaceport to unload. That is what causing them to take longer, not thier subwarp speed or thier hyperdrive.
And as long as you have a different cargo ratio between the freighter classes, then you will have different speeds due to different number of pick up stops.
So, again, subwarp speed, or even the type of warp speed drive, is secondary in determining the overall trip speed. The number of resource pick up stops is the primary driver in determining overall trip speed.