That's one of the most basic challenges of trying to design an "always profitable" starship using LBB2 trade rules and gaming out how much revenue the ship needs in order to break even. There is indeed a tipping point beyond which you can't reliably "get more" to fill your manifests with, meaning that extra capacity either winds up getting wasted every jump ... OR ... some of your capacity is dedicated to transporting through 2x jumps instead of just 1x jump for your next immediate destination.
So it can be done ... but there's more bookkeeping involved and it gets "spreadsheet complicated" in a hurry!
by the strictest interpretation, there's only one spec cargo per week, and only one roll for cargo. It's very limiting.
If one adds the bk 7 skills, Bk2 trade is much more profitable.
If one allows each crewmember, rather than each ship, to search, it's the 1000 Td ships that get to fill, instead of the 400 tonners ...
If one allows both, it goes even more profitable.
The less literal one takes the rules, and the less rule and more guideline one takes the rules, the less predictable, and the larger the ship that can fill at major worlds, and the more the difference between a small world and a big one...
How one approaches the rules strongly affects how the setting described in mechanics feels.
I always do my analysis by a fairly narrow "this is what the rules describe" approach. Any other approach is too many variables to be a consistent result.
CT has a huge problem in doing that, tho'... It has 2 variation on bk2, 2 on bk5, and 4 possible hybrids of using both; it has 6 personal combat systems (Bk1 77, Bk1-81, Snapshot, Striker, AHL, and the very minor differences between 81 and TTB/ST...
It has the duality of basic and advanced char gen, but only for a handful of careers. In theory, the Bks 4-7 versions aren't that likely to be much better, but the reality is that most will push their luck a bit harder in advanced, and the different rate of acquisition vs risk is muddled by brownie points, so Bk 4-6 characters are severely broader skilled, and Bk7 slightly more skilled. Aslan are under skilled.
There's no truly coherent view of CT from the CT era sources... the first truly coherent version is MegaTraveller, and that fixates the Big Ship universe, striker based combat and craft design, and Bk7 trade rules. I've never managed in solo play to get a MT designed freighter to turn a profit using MT rules. I was able to do it with Bk5-81 + Bk7, and with Bk2-81 without Bk5, and with T20. TNE I never tried it solo, and players needed patrons to simply keep flying.
It's because CT is a genre-engine, and designed with a mix-n-match toolkit mentality. The OTU was the afterthought that took on a life of its own...
One can easily do a total shakeup and be still running "rules as written" because of the modularity.