I aven't checked, but that particular violent bobbing I got with the nylon spacer in place felt much the same as with the elastomer. The elastomer is supposedly damped, I don't know how well, but that leaves the resonant system of the seatpost untouched, whatever you use down in the suspension. The way I see it, there are several resonant systems, you already called them, tyres, seatpost and rear suspension. Each has its own resonant frequency independent of the other 2. You have to treat each one independently for a given rider mass.

For the rear suspension, I think the blue medium Cane Creek elastomer I used for a while is close to being right.

The tyres I pump rather hard, so their resonant frequency will be high, out of the way.

Working on the seat post to lift the resonance...