How do I get pale sickly skin without it going chalky?

The flatness you're seeing is almost always due to the highlights being too stark against the overall pale tone. If you are going for a sickly Death Guard look, I'd lean towards a muted, desaturated green glaze rather than purple for the warmth. A hint of sickly olive or a very muddy sage works best to suggest underlying inflammation or disease, which anchors the pale grey-white and gives it life. It shouldn't be a vibrant color, just enough of a tint to suggest blood flow or decay beneath the surface.

Regarding your base, starting with a mid-grey mix is fine, but you might want to make sure that mid-grey has some saturated undertones in it. If it’s too neutral, the skin looks like plaster because there is no chromatic foundation. Try a light, heavily thinned wash of pale mint or seafoam applied over your mid-grey layer first. This subtly pushes the undertones away from pure grey and toward the sickly green/blue spectrum, making the final pale layers look like flesh rather than a mannequin.

For the transition work around the eyes and mouth, pre-shading the deep recesses is definitely the way to go for high-detail sculpt work. It's much easier to catch the negative space before you apply the main skin colors. Think of it less as a wash and more as applying a highly thinned, dark tone over the recessed areas to define them, like a subtle shadow map. Once those recesses are defined, when you apply the skin layers, the contrast will naturally clean up the harsh lines.

To handle those inevitable tide marks, which happen when washes settle in low points, you want to wait untill your skin layering is mostly complete. Grab a clean brush and a tiny amount of highly thinned pale contrast color or even a pale grey water-wash. Gently scrub or blot the edge of the wash pooling in the tricky areas, essentially lifting the excess paint while it is still wet. This keeps the shadows defined but prevents them from looking like muddy streaks.

I don't think adding that pale mint wash over the grey base is the solution. I found that changing the undertone with an extra layer of color makes the skin look too flat and applied, rather than organically sickly. Instead of running a wash over the top, I mix a very slight greenish tint directly into the initial mid-tone. I blend it thoroughly into the grey base before I start the layers. This way the green feels like a foundational color in the flesh itself, which makes the highlights look like they are truly catching light on a decaying surface. I prefer integrating the color rather than painting it on.

I've seen this struggle with pale skin tones a hundred times. You guys are overthinking the washes for the subtle stuff. When I tackled that lich piece last year, I didn't rely on liquid washes for the veining at all. I just took a tiny bit of a dark reddish-brown and stippled it lightly over the final pale layer. It gives the illusion of subsurface veins without all the pooling and tide marks you're worrying about. It's way more controlled than getting a wash just right. What size brush are you even using for the skin layers?

Quick update - I tried mixing a touch of that sickly olive green into my paint, and it really helped give the face some depth. Still, I am struggling with the deep recesses around the eyes. Do most people pre-shade those areas before applying the final skin layers?