.....sounds like the levers are not able to pull enough cable to make the brakes effective. I too would suggest Travel agentsFirst the bike came with V-brakes (Avid SD5) and matching long pull levers. There are no longer pull levers and a Travel Agent will only make the leverage worse, not better. Either the OP has the pads set too far from the rims at rest or the brake arm are too flexible or the pads are inferior.
Which brings us back to the condition of the wheel. I asked earlier if the wheel was true or not. If the brakes are loosened to accommodate a wheel that wobbles, they won't stop all that well. DaveB's implied question about the brakes being properly adjusted is fair game too. Even side pull brakes generally work. Cantilever or linear pull brakes should be fine for this application. Yes, a loaded bike takes longer to stop.
And again, there are hydraulic rim brakes for those that want'em. I checked, SRAM still makes them, looks like ~$600.
Pad are the cheapest change to make. Dir the OP try that?