Re: Maybe changing the braking system to the next model up?
I wouldn't go about messing with the brakes. You may well find that a 4500 uses the same brake components as a 3500. In nearly all cases the axles are capable of handling more than the manufacturer rates them at and often more than the tires are rated at. This also includes braking components which are frequently supplied with the axles to the manufacturers. No, I'm not encouraging massively overloading a vehicle, I think there is more forgiveness than you know. For instance, does your gross vehicle weight exceed the GVWR plus the manufacturer's allowable weight for an unbraked trailer? I wouldn't be surprised if Chevy let you pull a 2500 lbs trailer without brakes on it. That would be the same as stopping what you've built.
As for engine and other drivetrain wear...the candle that burns twice as bright burns half as long. There is no way around that. But modern technologies have made 200,000 or 250,000 miles the new 100,000 miles. Vehicles are better than ever.