Personally, I'd run LTs, simply because of their higher "reserve" capacity; upwards of 30% over the stated load. Given that STs have, at best, 10% (used to be basically 0%), you're still in ST load territory, with a much better tire. Hell, we used to run our old 1/2t trucks with massive loads and just air up to 60-65 psi and go. Yes, it wasn't very far, or very fast, but those tires still lasted 50-60k miles, usually with steel cord showing around the edges. :-) We'd then take them off and put them on a disk or trailer and use them until they sun-rotted.
I think someone miss-informed you about "Reserve Load".
All tires have a stated load capacity for example. "2,340# Max Load" molded on the tire sidewall at a stated inflation level such as "50" psi.
"Reserve Load" is the difference between the actual applied load and the stated load capacity and is many times stated as a percentage
Example: A vehicle is on weight scales and we learn that a tire has 2200# load on the tire. The tire has a load capacity of 2,750#. 2,750 minus 2,200 = 550# which is 20% of 2,750. It doesn't make any difference what type tire we are talking about as the math is still the same.
Now, it is true that for a given set of dimensions, e.g., 235/75R15, the stated load capacity is different depending on type tire and inflation level. P-type and LT-type and ST-type each have different stated load capacities at their stated inflation pressure. For this discussion, let's keep inflation differences out of the picture.
Let's look at a P235/75R15 at 35 psi is rated to support 2,028# ( In a trailer application P-Type must be De-rated by Load/1.1 giving 1,842# capacity.) An LT235/75R15 is rated for 1,530# @ 35 psi and an ST235/75R15 is rated to support 1,870#
BUT the "Reserve Load" calculation is still (Tire Load Capacity)/Measured scale Load).
The 10% margin for trailers is the difference between the GAWR and the total capacity of the tires on that axle at their max load. I have posted in my blog some actual margins showing that many cars have load margins of 25% to 35% while some RVs made before Nov 2017, when RVIA changed the "Margin" to 10%, had margins of tire capacity vs GAWR as low as 1%.
Hope this helps.