I agree, if you can get an E-rated tire you should do so. I wouldn't automatically inflate to 80 psi so get max load though, especially if you don't think you're overloaded at 65 psi. Running overinflated will get you more bounce on the trailer and especially if you don't have shocks on the trailer will bounce stuff inside around needlessly. You mention the closest scale is a junkyard, then I would say make a plan to stop at one on the highway on your next road trip with the trailer. Cat scales are easy to find.
I my experience all the wheels I have seen have a max weight listed, some have max PSI stamped on the inside of the rim as well where you have to dismount the tire to see the rating. I ran E tires at 75 psi on 6 lug aluminum star rims for years with no issues.
If you go to 16" wheels/tires you will likely need to lift the trailer. I went to 16" on the TT to run LT tires and wound up having to install the CorrectTrack kit to get an additional 2" of lift to stop the tires from rubbing.
