I'd disagree with the bias ply riding rougher than radials. When radials first came out the marketing hype on new cars was "radially tuned" suspension. This was a softer initial spring rate to absort the harshness of the radial tires vs the bias ply. Enthusiasts of course refer to the "harshness" as "road feel"
I too had thought the bias ply would eliminated the usual mode of tire failure with radials which is belt separation. Well, I guess strictly speaking that's true but a recent post showed bias ply tires can still suffer tread separation from the casing albeit with less dire consequences than a steel belt whipping around in you wheel well.
Ground clearance for me is a big issue and it takes some searching but good radial tires are available in the old 7.00x15 size. Of course support on the road is going to be an issue with special order tires.
I agree most of the choices are pretty bad. The market seems to be primarilly for cheap tires. Most people don't put a lot of miles on their trailer and since they degrade mostly from just sitting the major factor in buying new tires is cost. The tire manufacturers are forced to produce a tire with a very demanding set of performance characteristics at a rock bottom price.