S-ranking requirements are different for each mission and look pretty arbitrary. For the missions maybe destroying some more enemy units is the missing key, that was my issue when going for completing full s-rank missions.
I was so hyped for it too. I'm a newer stealth game enjoyer (Styx, dishonoured 1 & 2 no kill undetected runs), the original thief trilogy feels really outdated to me and was so hyped for thief 4. So damned disappointing.
Always has