I don't use sodium lactic, or add stearic acid or anything, and even my 80% olive oil soaps are rock hard once cured. My bars are never slimy, but I do live in a climate that is not humid and some maybe that plays a part.
I wonder if there are several things going on for you, if your bars stay soft even after curing:
As was mentioned, if the soap sits in water it'll get soft so look at getting a soap dish that raises the soap a tad.
Maybe it's not cured long enough? Even my soaps made with almost all liquid oils do get hard (and very hard) once cured. Of course, some of my soaps are 1+ years old. Make sure they are stored in a dry place, and not wrapped, as they will sweat. I have a couple of drawers in my laundry room that is cool and dry.
Recipe formulation- some recipes use oils like safflower and canola, which I believe can make a a softer soap (never used them, though). If you use even 20% of a "hard" oil, like cocoa butter, palm oil, tallow, etc, that can make a nice hard bar.
Have you tried a different
lye calculator, to see if that makes a difference? And make sure you use the correct type of oil (olive oil has several different "grades" as an example) in the calculator.
I don't discount water except in olive oil soaps, and I still have a very hard bar in just a couple of weeks for most of my soaps.