It has been my experience that a couple of lucky loads can finance the longer leggeds ships. The cost, however, really is that, speculating, they can not stick to a route. It's see what you have to buy, see if you can get it there, and then go there. If you can't get a trader roll for 1st die of 4+, and you have other options, don't buy.
Likewise, don't forget the use for Trader 1 (since it's worthless for jump-spec): locking down the first die locally before purchase. If it's too high, don't buy.
Note that under Bk2-strict, it's harder but still doable; always hire best available broker. Only buy stuff under KCr10 per ton base if the DM's exceed 6 on route.
Under Bk2+Trader Skill, don't buy unless DM's + 1st die difference is 6+ and still, avoid lots under KCr10 base. (You generally make more on KCr10 with differences of 6+ than the KCr 1 for cargo will provide. At base KCr5, you need a net difference of more than 20% base to make a profit; at KCr1, you need 100% of base as a difference.
Now, using T20... one can use a very-much book 2 methodology, and by use of trader and broker, and warehouses, make a route quite workable with J2 or J3 shipping on a route, and larger ships. But the transition to 2KTd freighters needs also the brokers and warehouses to make it work.