I don't know if your theory that 18's are better is valid. There's a lot of debate out there about using LT (light truck) tires instead of ST (special trailer) tires.
The ST's are going to have higher and likely stiffer sidewalls. I think they'll do better for the sideways dragging that happens when you do tight turns with your double axle trailer. The ST's are rated to 65mph. The LT's are probably rated for higher speeds. You'll have to really look and see what the load ratings are on each version.
the #1 reason why you'd go to 18" wheels on a trailer is for ASTHETICS. it's all about the looks. Also, the 18's are wider so boatmate needs to add 2" to the fender width.
https://rvingwithmarkpolk.com/2012/1...s-vs-lt-tires/
https://www.etrailer.com/question-59480.html