? on upgrading trailer tires from 14 inch to 15 inch rims

Well you could go up to 15", but Maxxis makes a 14" in a your size that is a much better performer than the Duro ST. Your rig's tires are properly matched to your axles and since tongue weight accounts for 15% of your rig's GVWR, the Maxxis ST tires will easily handle the rest.