It's not the size of the tire that's as much of a strain on the axle/bearings as it is the backspacing of your wheels - how much leverage your wheels exert on the bearings. With that said, the wide-spaced bearings of a spindle & hub assembly shrug off wheel backspacing much better than do the narrow-spaced bearings within a unitbearing. I don't have specs available off the top of my head for a trailer spindle/hub, but they'd be similar to that of a Dana 60, in which the spindle/hub assembly sees a 1.1:1 increase in bearing load relative to wheel backspacing, while a Dana 60 unitbearing assembly sees a 3:1 increase in bearing load relative to wheel backspacing.
Speed and load are going to be your trailer axle's critical factors, not tire size. Since you're nowhere near your axle's load limit, wheel backspacing is irrelevant; the larger tire diameter reduces bearing speed, which increases longevity and reduces heat. Don't worry about running 35's.