Possibly they should just take out the cheap Tritium from the tables and make Aluminium (high supply, fast regen) inexplicably sell for 40k/tonne in Natural Disaster state? There's nothing to say it has to be Tritium with the exceptionally high profit margin, after all.
That would give the same bulk trading profits, relatively similar galactic rarity, and not run out of supply.
Or they could make every trade good capable of profits in the 20-40k/tonne range depending on precise BGS states, and then any individual one will run low on stock in particular circumstances, but you just switch trade route and commodity at that point.