I won't do "trailer" ST tires at all if I can avoid them. I would suggest looking at LT tires, which may also come disguised as European spec "commercial van" tires, which will have a size looking something like the following, ending in a "C" which is NOT the load range, but a designator that it is a commercial and not passenger tire: 185/R14C. There are 15" sizes, more plentiful than the 14" example I gave. Not sure how available a North American spec "LT" is in 15" these days.
If you decide to go with a P metric passenger tire, the rule that I have heard is you must de-rate the stamped capacity by 10% for trailer use. I have no experience going this route.
My "ST" experiences have been as follows: a tent trailer with marginally spec'd 13" bias ST tires. Had me always more worried than they should be, but were a "better" Asian brand - Taiwanese company, but Chinese made tire. They held up until I decided to replace them when they reached 5 years from the code date, and had edges wearing bald. Did the upgrade to 14" tires in 185/R14C Load Range D, and these tires showed no uneven wear, and gave me much more confident feeling I was no longer pressing against weight capacity, low speed ratings, or relying on questionable construction.
The 5th wheel I more recently bought came with a very bad "China bomb" which are so bad, their name Towmax earned them a nickname in just about every forum talking about trailer tires of "blowmax." I even saw one of these blowouts when I went to get my LTs mounted, and another customer came in with his blowout. Here, I used a very common LT tire in LT235/85R16 Load Range E.
In both cases, the trailers were towed thousands of miles, for hours on end per day on the highway, with absolutely no issues.
If the trailer you are building will see any significant load, distance or speed greater than what a tractor might tow it at, then stay far away from the ST tires if you can. The older generation Goodyear Marathon had such a bad reputation, that I have no interest at all in trying out the new Goodyear Endurance.