The Trek 520 comes from the factory with 52-42-30 chainrings and 11-32 nine speed cassette. Ignore the high gears, the big chainring, because you don't have to use them. The not low enough low gears can easily be fixed permanently by the shop you buy the bike at. Tell them to install a 24 tooth inner chainring as one of the conditions of sale. Easy and simple. You now have low enough gears for touring. I'd also have them put on a 11-34 cassette instead of the 11-32 for a bit lower low. Again, easy and simple. Both these changes would be zero cost to you. Your gearing problems with the Trek 520 are now solved forever with no effort and no cost.
As for frame material, it does not matter. Many people have used the Cannondale touring bikes to ride across the US. Adventure Cycling's 2004 touring bike review article was written by a person who had an old Cannondale he had used to ride across the country and many other places. I suppose its possible the article writer with the Cannondale was trying to swindle us all into thinking aluminum Cannondales were good bikes when in fact they broke once a year. But I doubt it. I had a Trek 520 that was used many thousands of miles too.
As for cost, the Trek 520 and Cannondale T800 are both about $1100-$1200. Fuji makes a touring bike around $850. I suspect there may be a few others near this price range. But not many. I think the bike factories figure people who want a touring bike are dedicated enough that they will spend a certain amount of money. And apparently the bike factories decided around $1000 was the low end of what real touring bicyclists would spend for an official touring bike.
A hybrid would work for touring. Any bike will work for touring as long as you can carry or pull the gear you want to take along. Since you have a road bike now, see what it would take to use it for touring. Probably a triple crankset with the smallest possible inner chainring. And a rear cassette, or freewheel, with the biggest cog possible. You can buy rear gear sets in 6-7-8-9 speeds today with big cogs in back of 32 or so. You may need a new rear derailleur, $30. New chain, $10. New triple crankset, $50. Bottom bracket, $15. I suspect you could easily get very low gears onto your current bike for $100 at most. You need really low gears for touring.