I agree with Neverenough. The best way to better handle your trailer IMO would be to go with a good weight distribution hitch. Increasing the trailer capacity won't help on an overloaded 1/2 ton pickup. You might have plenty of power, but stopping and handling will not be up to par, even with trailer brakes.
It sounds like you need a trip to the scales with the truck empty, with just the trailer on and with it loaded. Adding more weight to the trailer by converting to a gooseneck will only make the overloading worse.
Your pickup has a gross vehicle weight rating which might be exceeded by a loaded gooseneck hooked up. The pickup also has a Gross Combined Vehicle Weight Rating (GCVWR). That's the truck, fuel, passengers, trailer and whatever the trailer has on it.
As an example: A truck may have a tow rating of 9,000 lbs and a GCVWR of 14,000 lbs. If the truck itself weighs 6,000 lbs loaded without the trailer, then the tow rating is reduced to 8,000 lbs plus the truck weight of 6,000 lbs to not exceed the GCVWR of 14,000.
You mentioned this individual welder has done quality work for the city converting their trailers to Goosenecks and that they haul backhoes and everything else. What GVWR trailers did they start with and I bet they use at least a 3/4 ton truck to tow them.
Where you live, the laws may be "relaxed", but you can't cheat the laws of physics for long and get away with it.
Your project sounds interesting, buy your safety and that of those around you are the most important thing.
Good luck