The threshold for Paul-L’s de-esser should be close to the RMS of the audio, (+/-2 dB).
(IMO leave the other De-Esser settings as shown).
Paul-L’s de-esser is a multi-band brickwall limiter, rather than multi-band compressor,
so it can sound unnatural when removes a big chunk of loud sibilance: it shaves it off flat.
A workaround is to create an in-sync duplicate track of the original,
and mix that in with the de-essed version, at about-20dB below the de-essed version.
Then the de-essed sibilance then has some variation to it, rather than shaved uniformly flat.