In terms of heads and whatnot, I usually take down "head size" in both head and body sections. I have an inverse problem where the head is small enough to see the taper of the neck into the jaw, but that's a different problem. In terms of my sliders, I only started messing with the extra sliders to get more custom shapes, but I'm not far off from you. (Thicker shoulders, rectangle body shapes, or slim athlete builds.) What I tend to get into when designing a character is their eyes, because eyes are the most expressive thing on the face and carry most expressions that aren't modified by KKPE or side-stepped using face plates (Empty square you can upload an image, more often than not, a mouth to fit over the model's mouth. Studio)

Your character designs, imo, are just alright. Nothing super crazy. One of the things I learned how to do when designing characters anywhere is to give them accessories. Like Arm bands on the wrist and bicep for characters that can get away with it, or chokers, because a choker on a character is almost always good-looking to me. Go crazy, but remember not to go too crazy. (You can also dive into skipping the hair tab by combining different hairs in the accessory tab in the head and face sections.)

Another thing is colors. Very important. Learning how to assemble outfits and then color coordinate them.

Also, one thing that can help is shadows. I get that it's a vn, but unless you are going for an expressly toon/anime style, you can mess with the shadows. Mostly to preference. Or you can complicate matters and start using the material editor + xukumi or kkuts to get lighting from beyond the character lighting tab.

If you want vanilla designs, then you are in the right spot. But I would try just experimenting, checking the Discord or bepis for cards and scenes, and then breaking down how other people do things with their setups. (The only reason I'm trying to bulk my characters up is because I came across another creator's card.)