Another Audacity tool I find can suppress harsh, noisy “S” sounds is Noise Removal.
Close to the bottom of this thread… best way to improve old audio
…shows NR settings and mp3 sample results that delivered total dead air silence to vocal recordings that had a lot of noise…
You’ll have to tweak to get the best roll off between actual voice timbre and “S” sounds. Record or find a mid intensity “S” sound in the audio stream to establish a noise profile and tweak to get the best results.