You say you ran the de-esser. Maybe a little too much because in places it sounds as if you have a speech defect.
Hi, Guenter.
You've got good ears!
I've got a speech defect called a "lateral s". Unfortunately, it's based around a physical condition that no one who knows me is likely to believe:
my mouth is too small.
Well, for my tongue to properly form an "s", anyway.

If the vocal were multi-tracked, I'd have been able to correct the "s" with volume envelopes. That obviously wouldn't work here, but you've got me wondering if an EQ notch filter would have any effect. That's basically what a de-esser is anyway - I was just in a hurry and after twiddling the knobs simply gave up.
Thanks for your attention to detail!