IMHO, most people don't "decide" anything. They just blindly forge ahead with no idea that they're overloaded. Blissfully ignorant.
THAT is how they "get away with it."
You are correct, going strictly "by the book" you do not have all that much tow capacity with your current truck.
Even though you only have a "payload" of ~2000lbs, take a look at your axle weights.
The rear axle only has 2900lbs on it empty, and it has a rating of 6084lbs with stock tires. That give you 3184lbs of axle capacity. A gooseneck is generally installed directly over, or maybe 1-2" ahead of the rear axle so almost 100% of the gooseneck weight will go on the rear axle.
Looking at it this way, ignoring GVWR, opens up your towing capacity to about 15,000lbs.
You will want to register the truck's weight as the sum of the FAWR and RAWR, not the factory GVWR, if you decide to go this route. The potential for prizes (even just trophies) at the horse shows makes you "commercial" in some states.
