Trailer tire questions

Don't compare ride quality of an ST tire to a vehicle tire. They are entirely different animals. Try not to read too much into a "harsh" ride for the trailer. ST tires are made for trailers, they have stiffer sidewalls and a built to support the weight. The trailer is built to sustain the bouncing and stress on the road within normal use. The construction is somewhat fragile yes, but only if do things to it that it wasn't built to do, like try and take on a rough off road trail.

Check your manual for proper inflation. I've yet to see one that didn't recommend anything other than what it says as a max on the tire. Mine says 50 psi as does my trailer's manual.