Syllabic squelch is the only good HF squelch.There is a squelch in Linrad that is based on an FFT
analysis of the baseband. It works in all modes,
AM, FM, SSB and CW.
It would open the squelch at very low levels and the user
can set a delay to not loose the first fraction of
a message. Those who have an interest could make a wideband
recording available to me and I would make a video on how
to use the Linrad squelch.
Linrad is free software. Open source, with the most generous
licence. The squelch is a simple procedure that others could
implement. I think it is close to optimum.