I'm putting about 10,000 miles a year on mine. And this isn't just one item or instance. A lot of stuff comes from China. Some good, some bad. I can say the same about domestic stuff as well. I'm a little confused in that you say you've had no issue with brakes, but that you sent back the defective parts that came with the set you installed...
No Colby, I don't tow over 10,000 miles a year--one year I did tow about 8,000 miles. I have been towing boat trailers since 1956. Not many folks in their 80's do a lot of boat trailering.
Not sure why you are confused. It was obvious when look at the castings, that they were not machined properly. So I sent them back before I installed them. I said specifically E/H brakes--I have had issues with surge brakes.
I rarely see goods made in China which can equal what is produced in the USA, Germany, and a number of other places. For instance camera lenses, knives, etc. Much of SS from China, rust, and are really not really 316. Often Chinese junk looks as good as the ones built in the US, but they don't have the longevity.
It sounds like you are properly tensioning the spindle nut. The way I do boat trailers is to torque the nut to 50 ft/lbs, to be sure the hub/bearings are well seated on the spindle, Then back off about 1/4 turn, and then tighten finger tight, checking to be sure there is NO end play. The wheel should spin freely. Also watch for disc/calipers drag.
I am surprised you don't use tire pressure and temperature monitors on your wheels. (Since you have a newer truck, it should have the tire pressure monitor and built in for the correct trailer sensors.). The TPMS is not all that expensive, and can be life saving let alone saving damage to the trailer. I also stop every 2 hours, check trailer hitch, State of the motor, and all straps and tie downs, as well as temp of discs or drums and hubs with an IR thermometer.
I don't believe that there is enough scrubbing effect with a 2 axle trailer from those times, when you are making sharp turns to cause any brake issues. In any case, there should be no play on the bearings/spindle/hub to cause brake issues.
I don't even pretend to know more than the trailer professionals I use. They have 3 mechanics working full time on repair of trailers--often brake issues.