[Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

[scc]

OK, I'm working on a so far hard Sci-fi setting and I've got two planets that are 400 AU apart, +/- 44 AU. Now according to my math that means that at a velocity of ~300 mps it will take a ship 4 years to move between the two planets.

My problem is that I want there to be somewhat regular trade between them (In fact one's supposed to be settled from the other).

Now the basic issue here is drives, the tentative TL for the setting is TL9, but I'm thinking that most of the freighters use older systems, such as Ion Drives.

No I'm thinking that lithobraking can be used at the journeys end to slow the ship down, but that still leaves me trying to figure out how to get the ship moving that fast.

Boosters/upper-stage's are just sort of wrong for the setting, there's supposed to be a lot of traffic so it doesn't work out. Catapults could work, but how long would a human usable one be? Increasing the amount of delta-v is out, at least for Ion Drives, from what I can see the specific thrust they get is Spaceships is twice as high as it should be. Mag-sails are an option, but both they a Ion Drives have an acceleration problem, though I might be able to get by that with Mag-sails.

So what do you suggest I do?