Did search on this topic and spent a long time but found nothing specific to my questions.
Have to replace the Loadstar 4.80-12 Load range C tires on my boat trailer. Found worn tread and "spider web" cracks on the sidewall. Tires are only three seasons old- maybe 2K miles on them. I thought I was inflating them correctly; they are always on concrete during storage. Rated for 90 psi cold inflation pressure under max load. My 1648 jon with longtail motor, gas tank, battery, and hunting load I estimate around 950#, so C load range might be overkill but want that margin for any extra weight. Cost between B and C range is only 10$ difference per tire.
Been shopping- Kenda, Carlisle, Wanda- read bad reviews on the Carlisle's. Any recommendations?
Given my weight/load, and if the cold psi rating is 90 psi, what would you inflate the tires to?

in the past, I have raised the fenders and put on larger sized tires on boat trailers. as long as u use same bolt pattern it will work. bigger tire the better.

or just get new axle/tires and raise fenders so tires don't rub.

Use is the key, your old tires "dry rotted" causing the spider webs due to lack of use. 2k in 3 years is nothing. Psi is per weight, you don't want so much pressure that your riding on the middle of the tread. You can chalk the tread or simply ride down a dirt road and then on the pavement to see what's touching. Balance is a major factor that most folks overlook with trailer tires. Balance them and they'll last. Plus, damn a "trailer" tire, get a regular radial for a car. Or Goodyear marathons.

Believe you me, will, I wish I could use the boat more! I was using an average of 25 duck hunts to the local wma- maybe 35 miles roundtrip for the last three years...I did forget to add half that number of fishing trips, so maybe 3,500 for the last 3 years which is still not much.
Would going to a Load range B bias tire improve longevity of tires? The local shop that mounts the tires- my galvanized wheels are still good- does balance the tire/wheel combos. They inflate to 60 psi and total weight per tire is 795# so total for both is still about 500# more than my loaded 16' jon boat.

I steer away from the bias trailer tires, always had wear problems. A good balanced radial is all you need.

Glades, I’ve been told radial trailer tires are best for highway running because they run cooler. The bigger the diameter tire the cooler they run ( less revolutions per mile ).They are all glass belted vs steel because Trailer tires sit for long periods of time so fiberglass does not hold a flat spot easily. Also the reason for the 65 mph limit. Because of the cheap galvanized rims you can’t balance the tire. All trailer tires deteriorate from UV light. Does not matter if in the sun or under shade the UV light still affects them. Half life of a trailer tire is 3 years. After that replace no matter what shape they’re in unless local use only. This info is what I’ve learned over the years but maybe someone with more experience can confirm it.