DOWNgrading truck - sanity check please

Goodness no. The specs on that trailer are a match made in heaven for any half ton. The LT tires should be rated for far more than 35 PSI. If not, they are P tires. I ran LT on my half ton with 55 to 60 and it made a huge difference. Check the sidewall rating and be aware the rims if not rated for an LT won't cope with full PSI- I believe it's the valve stems.

I can't see a single issue with what you are proposing.