A lot of steering damping comes from the weight of your hands on the grips. Some bikes will shimmy if you take a hand off, even if all the bearing adjustnents are right. If you have a folding stem, would it be feasible to adjust it so your bars come back a fraction (if it can be done without upsetting the fit)? This would have the effect of fractionally increasing the tiller effect, and might reduce or stop the shimmy. It might not, but could be worth trying. Even slightly wider bars, if yours are particularly narrow, could help. You also could try riding with your hands further out on the grips at times that you think the shimmy will occur, to see if that has an affect, before spending money on changes.
A bit of lateral thinking. A few years ago I fitted a sidecar to a motorbike I had, and the steering damper that came with it was shot. With standard bars the front wheel would shimmy alarmingly at low speeds. I tried fitting wider bars and the shimmy disappeared as if by magic, and I was able to dispense with the steering damper. It seemed to be due to the leverage and weight of my hands on the wider bars, and involved no effort on my part. It just ...worked!
So it might just be that a simple, seemingly insignificant adjustment might do the trick, no matter how illogical it might seem, before you get into taking things apart, fitting O rings, etc.
