You don't give yourself enough credit! These are outstanding and I can tell that a lot of time and work went into them. I've done some art in the past and one of my struggles in some of my art has been not completing pieces as I get bored and wish to start something new and (possibly) better. I'm a person who jumps from one thing to the next and lately I've been mostly stuck on wanting to be a musician someday.
To start something and see it to the end, this takes great patience and resolve! You've really inspired me with these! You have a beautiful depiction of Astaroth as well. Did you use any reference photos or anything for it? If not, that's just that much more impressive since drawing people is very challenging.
One small tip might be to gradually add some more darks (almost black) in tiny areas for better contrast. Also, for better dimension, it may help to try visualizing where the light is coming from. In this case it looks like a upper-right straight-on kind of an angle, so it might be best to remove a bit of the shading at the top-middle part of the forehead and the bottom-right part of the jaw with some highlights around the middle part across the chin. Also, just a bit of shading on the white parts of the eyes (very light and narrow on the left with a bit less on the right and an even smaller touch on the top and bottom) might also might help things to pop out more. Some light on the cheekbone areas might help with some shading coming out from below the inner-corners below the eyes (more on the left than the right) might add more structure, along with shadows cast from right-to-left (from the hair and nose). I'd recommend not going right to the eye though (just barely below it) leaving that round shape of the lower eyelid whereupon you could add darker shading followed by absolute light going upwards.
To get a rough starting point I like to draw in faint lines where I think the shadows should go and work from there; in this case, two narrow sidways triangles pointing away from the nose that curve along the bottom (for the cheekbones) and the top (for the eyes) with the bottom part forming a long narrow point alongside the bridge of the nose. It's all about trying to see the shapes and topography.
It always helps to look at random photos too. I'll add one that I think might help out.
That said, it's a beautiful piece of work even just as it is. Art is subjective after all and I do apologize if my response comes across as too critical. My hope is to keep it more on the constructive side.
I'm also curious about the coloured pieces. Are these in pastel? Or coloured pencil? They are fantastic!
Keep up the great work!
PS - Image doesn't attach well (it shows errors) so I added it here instead just in case:
You used to be able to use brackets with [ img ] ... [ /img ] to post it though I'm not sure if that still works. (Worth trying out).
Sorry for my slight OCD there!