Well, I'm at the other end of the spectrum, I wouldn't have a bolt together trailer (although I did for my lawn tractor years ago) to go off road or even unimproved roads.
Welding is an art, you need to find an artist. A good weld will not fatigue any more than the metal, it's all about the prep and finish work, the type of weld and the engineering for type of material, thickness and build specifications.
Mild steel is best for rough abuse, most opt for heavier aluminum to save weight. IMO.
Never had shocks on a trailer, seems it would help, but our old travel trailers just had spring axels and the plastic ware inside didn't break.
When it gets really bad on washboards air down and let the tires soak up the bumps.
I recall my John Deere going airborne after hitting a bump with the trailer I mentioned above, just had the front lashed in and that happened in town!
My utility trailer is basically the same design as OverlandNA's above but with a steel plate floor, expanded metal on the sides and a larger ramp gate, it also tilts.
I need to do something with that trailer, it's just neglected now that I have 5 trailers!