i have some tutorials and process videos on my tiktok, including a tutorial playlist, and some stuff tagged on tumblr but i don't really have like a definitive guide. curls are VERY diverse and every head of curls contains multiple curl types. understanding them isn't just about learning to draw them, you have to learn about hair and how it behaves and for my part of that comes from having naturally wavy hair that sometimes ringlets (even if i don't wear my natural texture anymore) and watching a lot of videos about curly hair and gathering a lot of references.
also, when i say i draw every piece of hair--i do not mean individual hairs, but rather clumps. curls form clumps and the size of those clumps is a big factor in the over all determination of hair texture. straight, wavy, curly, and coily hair actually have different follicle shapes and these determine the category of hair texture, but a lot of the tightness and definition in a curl is determined by clump size. the smaller the clumps, the tighter the curls are/look. i have a looot of hair references saved that i use constantly but i don't just look at them--i think about how that texture and definition is achieved. when i can, i make an active effort to find the hair care routine of the person i'm referencing so i can see their hair in various states, including styled, brushed out, wet, and with different (and preferably also with no) products.
so when i think about defining hair visually, those are the main things i think about: texture, care, and clump size. and light. you can broadly think about different types of wavy, curly, and coily hair as either cylinders, cones, or ribbons (and ribbons can also be used for straight hair).
for most textures, the more defined hair is, the shinier/more reflective it will be, and the more light it will catch. in general when shading a full head of hair it just comes down to practice, observation, and trial and error. the hair that is in shadow will catch Less light but very infrequently will it catch No light.
i think the problem you are running into is that curls have a lot of dimension and shape so when you separate it only into Front Pieces and Back As A Single Mass, you are basically flattening it into a couple cardboard pieces instead of seeing it as a complicated mass of smaller shapes that all interact with each other, similar to a mop head
i recommend taking some time to study some references, including drawing from them and trying to break down the hair into larger shapes. try this several times with different categories of shapes--whether the shape is defined by mass. when you do this you'll notice that even the clumps of hair that catch the least light still usually catch some light.
it's also important to note that most light is caught on the parts of the curls that face outward (or towards a very directional light source) and that the parts where curls face each other is usually in shadow. it's part of why curls are so hard to simplify.
i don't really have time to go more in depth than this so i hope this helps!
and i guess maybe the biggest takeaway here is that if you post your curl routine online with good lighting there's a decent chance you'll end up in my reference folder lol