Sizing charts are at best estimates. You need to actually get on a bike and ride it to know for sure, as people of the same height might be proportioned differently. Some people have longer torsos, some longer legs or longer arms. And this affects how the bike will feel when riding. And fit matters. For years, I rode a bike that was too small because my legs are short relative to my torso. (if my legs were proportional to my torso, I would be around 5'11", but since they are not, I am 5'8"). So I found I need a larger bike for reach, and have to live with less standover than I might otherwise prefer.

Second, brands. Yes, you are ignoring all the brands except for Trek, and you really shouldn't. It isn't that Trek is bad. They are a solid choice, and the size of the company and dealer network is probably the best in North America. (and that does mean you should have no problem selling your bike in a few years if you wanted to) But they aren't the only brand on the market by a long shot and at the level you are looking at, just about every brand has a product that competes favorably with the FX line. examples include the Giant Escape line, the Specialized Sirrus, the Jamis Coda, the Cannondale Quick, the Kona Dew, the Fairdale Weekender, and no doubt, at least a dozen other quality products that I failed to mention. Most bikes retailing for $400 to $700 are spec'd at a handful of factories in Taiwan and China anyway. And the components on these bikes are similar if not identical. It is likely that if the plant was making frames for Trek, also made frames for other brands.

Finally, if you are actually looking to do rides of 50 km (30 miles) or more, you might want to consider a true road bike. I know some newbies are afraid of road bikes, but if you actually plan to ride some distance, drop bars are far better suited for it than are flat bars. There are people who do long rides with flat bars, but worth considering stretching your budget and just getting a road bike.