Exactly! For instance, we can ask ourselves what characteristics the ideal Trade system would have in Classic Traveller. The answers will NOT INVOLVE realism to very great extent, and will not involve economic simulation AT ALL. The main successful traits will be that the system produces marginal gains for the PCs involved, in fact that it should, ideally NOT be sufficient to support them, certainly not in any style.
You've been playing it poorly, then. It can, for a pure cargo J1 design, make reasonable money. Passengers are a liability and financial drain. If you get a ship in CGen, sell it. Use it as capital to build a pure-cargo 300 or 400 Td Bk2 ship, and it's going to average a few hundred credits per cargo ton.
Use J2, and you triple the number of destinations. Always go where the mods are best. Always use a broker.
Short routes can be generally profitable to the level of Cr100 to 200 per ton, after salaries, op costs, monthly payment, fuel, and docking fees.
A pure cargo J1 >199 Td ship can make do on freight alone; costs are Cr800 per Td on a 14 day schedule.
Add a factor at either end, and a warehouse, and you can get this down to Cr700 per cargo Td... via a 10 day schedule. If doing LASH the costs are a bit higher, but you can get a 9 day schedule... and push the costs down even further. You do, however, need 3 lighters for one ship on a good run.
Two system LASH route:
Lighter 1 is on ship, Lighter 3 and ship on world A; lighter 2 on world B.
Week 1: ship (with lighter 1 full) goes to world B. World B factor loads lighter 2. Factor at A looks for a bargain.
Day 8: ship swaps lighter 1 for 2 at B.
Day 9-17: ship jumps for B, Factor at A fills lighter 3. B Factor looks for bargain at B
Day 17, ship arrives at A, swaps lighter 2 for lighter 3.
day 18 jump for B
Day 26, swap for a refilled lighter 1, leave 3
Day 27, jump for A
and so on. Note that the 8th day is to allow for the listed time variability, the extra day for loading the fuel, doing the nav.
It should largely prevent them from settling down into a fixed 'trade route' or schedule, since that would become dull and routine, and inhibit the exploration of the Universe that is a thrust of the game.
There's a JTAS article about how to pick your route.
Now, if one is willing to tramp, sooner or later, under book 2, you get a good run, and instead of a "pay the bills and grow the speculation fund" you get a sudden large influx. I've had several cases where crews worked up to a couple MCr in the spec fund, and that with each PC getting one shar of the profits, the ship's fund getting 2. Do note: one needs about KCr100 to be able to leverage the speculative trade effectively for a Type A; about KCr150 for a
If you are using B7 characters with Bk2, Trader skill 1 allows you to see 1d of the 2d on the Actual Value Table before committing. This makes it far more likely to not sell at a loss. At Trader 3, take the couple hours to find out which destination has the best sum of 1st die roll and modifiers.
If using book 7, you don't have the spikes in base value, due to the uniform price, but the margin is, with broker 1 and trader 1, often about Cr1400 gross increase, for about Cr300 per ton profit. Bk7 is the default for MT, TNE, T4, and T5; T20 is a Bk2 variant. GT is its own thing, and GTFT is yet another thing. MgT (Mongoose) is yet another thing unto itself, but like Bk2, speculation and tramping maximize profits.