If the dust line is showing that you're using all the travel, but you still feel harshness on small bumps then it just be the valving. My XR400 was like that and it was no improvement running the thinnes oil I could find. When that didn't work I started removing shims from the compression stack and still had no improvement. I finally sat down and inspected the valve itself. It was so small, and described designed so poorly, that I ender up remachining it to get more flow through it. Stock, it was hydro locking.
That's the true advantage of USD forks. The valves in the XR were about the diameter of my ring finger. The valves in modern USD's are nearly the size of a goofball. The get that big of a valve, capable of lots of oil flow, a conventional fork would be massively large. I need to do some research. I'd love to see what the inside of your forks look like.