I must have been unclear, I wasn't suggesting this was bad advice about learning to ride off pavement. But it you are going to have one bike, a 250 dual sport will be horrible to get you to big bend from college station. A cb500x will get you there much better and also give you a chance to practice off pavement closer to home without nearly as much risk as a big adv bike.
There's just no really perfect compromise. I have zero experience on a bug adv bike but my limited experience on my compromise bike says practice is critical to success off pavement. Having a bike that will get to off pavement to practice on is job number one. You have to practice on the kind of bike you will be riding once you arrive at big bend.
Hence my suggestion for a small folding trailer or a hitch carrier.
I'd also say that a CB500x would also be fairly not pleasant in getting from anywhere to Big Bend, but that is just me.
And if you are going to get a CB500 which weighs 430lbs, why the heck not just get a KLR650 since you are at the same weight? Either one of them are heavy, and will instill bad habits, so you might as well have a 21" front wheel and 7 gallons of gas if you are going to have the same amount of weight, right?
I will say, that the advice of "you have to practice on the kind of bike you will be riding once you arrive in Big Bend" is what leads many many people to learn some pretty terrible habits. If I want to start road racing motorcycles, I don't hop in and get a 1000cc supersport and hope for the best. I learn on smaller and more forgiving bikes and then move up as my skill level increases. For some reason, people don't seem to stick to this same logic with riding off pavement. They think it is so much easier, and since you are going slow it isn't like you will get hurt. I can say from my experiences that it is far more often that I see folks new to dual sport get injured, than I do with my buddies that are pushing the limits learning to road race. Perhaps it is the common respect that most of us seem to have for road race machines and the HP they have.
I know this isn't an easy answer for the person that posted the question. "Oh great, now this yahoo on here is saying I need to buy some little crap bike that doesn't even do what I really want to do, ride that for a year, sell it, and THEN finally get the bike that I should have gotten a long time ago." Yes, that is exactly my advice if the end goal is to learn the proper way to ride off pavement, and to do so with the shortest learning curve possible. Or at least that is what I've seen in my past 35'ish years of riding these things. But like I said before, we all have opinions, so who the heck really knows.....