b-dog...
You can do everything right and still have issues with many brands of china made C/D and E load range ST tire bombs. Hell we had the same issues with them when all ST tires were made in the USA.
Sounds like you are doing everything right.
Having worn out dozens of tires (five trailers on the road with 28 tires on the ground) on trailers in my business for a eleven year straight period I've found balancing a tire (P/ST/LT) on a trailer wheel, off the trailer, is wasted effort. If it needs balancing spin and balance the wheel on the trailer with the old style machine and match mark wheel to the hub.
Most tire tech issues with the tires on or tow vehicles do not apply to tires on a trailer.
That why I use truck tire dealers in my area are more into dealing with commercial customers and tire issues on a trailer. Both of the one's I use have been in business for 50+ years and still use the old machines on trailers with the size tires in discussion with lug centered wheels.
Check out the Providers....Endurance.....Carlisle HD I mentioned above. The Providers have been around since the '10/'11 era and have time and miles reliable service record behind them. The other two have been out 2 years or less and don't have a time and miles of service yet but both are popular. All three would be a better choice than any of the old class 65 mph rated C/D and E tires.