Which, to me, suggests most of the "ore freighters" of the TOS era would probably be interplanetary spacecraft, not interstellar ones. Refined materials would be shipped by boomers of the cargo service since they are more valuable, but unrefined material probably goes out over short-range robot ships.Well, nothing makes it profitable to haul ore in modern times. Which is why nobody does it, not over oceanic distances. (It's only hauled a very short distance to a refinery where it ceases to be ore and becomes, say, iron.)What, exactly, makes them profitable in MODERN times?
I haven't seen many references to that, although I would say that in a system like, say, Alpha Centauri that may not be such an impressive feat as you would think.This is among the most interesting aspects of TOS space trade: the explicit references to ore being moved from star to star.

Hardly. I'll again draw attention to the Janus colony, which is described as being very geologically rich to an extent that a "second Earth" could never be. More to the point, Janus is otherwise uninhabitable and isn't very attractive to anyone EXCEPT for its mineral wealth.But all the colonies are virtual Second Earths, as far as we can tell.Although you could purchase commodities and raw materials from various traders, you would always be subject to their prices and their trading limitations. If you obtained these items through your own colony, then you possess them and control their availability. If you have more than you need or wish to keep in reserve, then you can trade it and make a profit.
I would speculate that "garden planets" that are very Earthlike are probably valuable for mundane reasons (e.g. realestate) in which case the residents would have to purchase almost EVERYTHING from other colonies that actually manufacture things. They'd have to pay for those products probably by work either in the service industry and other intangible goods, or by physically shipping out from the garden world and doing the hard labor in some of those other less hospitable/high value asteroid colonies.
I don't know that "farming colonies" would actually be all that profitable in and of themselves, but would probably be extremely necessary for Federation planning and would probably rely almost exclusively on subsidies.