I personally hate pulling a trailer with bias tires..
Every pebble,every tar strip,every joint .. becomes a vertical bounce for a trailer equipped with bias tires,primarily when running lite or empty.
I think you will need to stay with the ST tire as it is very unlikely to find a 15" LT tire rated over 6 ply.
If your trailer gets very few miles & the tires always rot b4 wearing out & the price is greatly more for radials,then you might weigh all that b4 deciding .
If you use your trailer for longish trips rather than short hops....
I just bought a new trailer to haul lawn tractors & other equipment.
It had bias,i insisted that radials get installed.