I would ride a lot of bikes, as many as you can try, and keep your mind completely open about the type of bike. To stick with the Trek brand just as an example, their FX line is awfully varied. You can buy something for as little as about $500 or one costing many thousands of dollars. They'll ride similarly, though the nicer bikes will have nicer components. Will that make a practical difference in your case? That'd be hard for us to know. We don't know your location, and you haven't mentioned hills. Not sure if you live near the beach (mostly flat) or if you travel there often and might also ride where there are hills. Trek also make the DS (Dual Sport) line of bikes, with front suspension forks. Trek also make the Verve, which is more of a comfort-oriented model line. Each of these has specific advantages and disadvantages. Ride them all to see what you like the best.
I think the point that "Colnago Mixte" is making is that it's hard to buy for the future, not yet knowing what you may grow into. If you think you'll grow into drop bars, just buy a drop bar bike at the onset, and learn to like it, right? I say that somewhat in jest, but it is indeed hard to know what you'll end up preferring, or growing in to. You may buy a cheap bike (new or used) today, decide two years from now that you want an expensive drop bar bike, and go do it. You may also buy a pretty expensive bike today (like an FX Sport 6) and still decide two years from now that you want an expensive drop bar bike.
Go in knowing that your desires may change. They almost certainly will, given this is your first new bike purchase. Given that, I'm not sure that I'd start out with a $2,000 carbon fiber bike as my first new bike, only because you've said it clearly enough that you're not sure what you really want. To your desires of "smooth" and "going to last", a $500 Trek FX 2 certainly meets that criteria. Or consider the FX 3 ($660 with rim brakes and $780 with disc brakes). The FX 3 has a carbon fiber fork, which may give you a more compliant ride. Definitely ride them though -- check out Treks, Giants, Specializeds, Cannondales, Fujis, Jamises, etc. Anything you can test ride will add more knowledge to your bank, and may help you target what you really want before making a financial commitment.
Welcome!