Your trailer came with 15" load range C tires at 50 psi.
When upgrading tires from a 50 psi rated C tire to a load range D requires a rim that can handle 65 psi. Some rims will and some won't so make sure your rims are rated for those pressures.
Same with moving up in tire size. Some 15" sizes of C tires require a 5" wide rim. Moving up in size may take a 6" wide rim.
Many times upgrading tires size and pressure rating requires new wheels. ST tires are good for 3-5 years so I would go with the proven Maxxis ST8008.
The best long term upgrade on your 9350 lb 5th wheel trailer is the LT E at 2680 or 3042 lbs capacity. I run them for 7-10 years depending on which trailer type and 50k-60k miles.