Not many 15" LT tire sizes left anymore with 1/2 ton trucks going to 17"/18"/20" tires.
Your ST225/75-15 C load range/50 psi tire has 2150 lb capacity.
I would go with a ST225/75-15 D load range/65 psi at 2540 lbs capacity for a big 20 percent reserve capacity. This size tire is very common and available just about anywhere.
The Carlisle RH or the new HD.
Provider
Goodyear's new just out made in the usa Advantage ST tires.
Maxxis is another option but not speed rated same as the new gen ST tires and not all sizes have the nylon tread cap overlay.
You don't need a E load range 15" tire on that small trailer. Some 15" wheels aren't rated for 80 psi so check them if you go that way.