I purposely drove harder / more wreckless than I normally would, taking hills/mounds at WOT and trying to jump it around as much as possible. I'll pay for that tomorrow. The welds all look fine. I also pushed this thing across the yard by the rear rack and it feels stiff, with no flex, which is what I wanted.
I figured out why the belt was sticking. The clutch was partially engaged. So that's lubed and it now idles and accelerates as it should.
I realized today why the brakes have always felt a little to squishy. I'll throw a new band on tomorrow and I'll give the kiddo some time to abuse it before I paint the bare metal. I think this cart is almost ready for long term, low maintenance use.