So looks like I finally figured out how Stealth works… the lack of movement is the trick.
I’m only at 4/5 for both Stealth and Sneak attack perks, but when I’m stationary and there’s no light source, I’m at 0 detection. Zombies will literally go up to my face and not attack because they don’t detect me.
If I prep properly and wrench/pickaxe all the light sources in a main area, even the triggered sleepers don’t detect me. Obviously they wake up, but I can still sneak attack them or at least be relatively safe even while they run around.
I also always get Parkour, so my backup would be setting up platforms 2/3 squares off the ground so I can jump up there and hide.