First; your drawings are very, very good. I would be delighted to see more of them. Never stop with pen and paper.
Disclaimer. What follows are for the most part my opinion and personal experience. And pardon me for going on a rant about context.
Background, scientific illustrations
There are still people employed and being educated as scientific illustrators. These people work for the most part with pencil, ink but most of all; watercolours. Particularly in the life sciences there is a big need for scientific illustrators, and there are good reasons for this. Many highly educated in these sciences are also excellent at drawing, sometimes accomplished artists, though the full-colour illustrations they leave to professionals.
Palaeontologists draw their fossils, archeologists draw their digs, some neuroscientists draw their neurons.
There are reasons for this, perhaps the most important being:
Nothing – photography included – can replace the seeing. Drawing or
painting something forces you to intensely see. The brain is a
wonderful thing, but we "fill in the blanks", we assume, we create
patterns and can therefore loose important information. In
identification of species, fossils etc, this is a serious mistake.
Personally, I draw stuff, I have the luxury of cutting corners. I draw a foxglove, and if i skip a bud or a leaf, or if I place a leaf slightly more to the left, no harm done. No one will ever know. My brain makes assumptions, and when I realise that mistake I do not have to correct it. A professional scientific illustrator do not have that luxury. She will stare so intently on every tiny little part, every shade, texture, shape, and so force her brain not to make assumptions. Simple example: butterflies. Glance at a butterfly and you would say it is symmetrical. But they never truly are, not in wing markings, wing shapes nor colours. Not entirely. But your brain will jump to conclusions and assume. In scientific illustration there is no room to assume.
These people paint from specimens. They can touch them, turn them over. They are able to recreate extremely subtle textures in tiny details that would be lost in photography (or, you would have to take so many photographs under so many different lighting conditions, that the whole idea of an identifying specimen just falls flat).
Your work falls into this tradition.
The value of your drawings
From the examples; they are splendid. They have, to quote @plainclothes humanity. They have personality, and in a way, they feel more trustworthy, more consistent. You spent time seeing. The drawing of the eye: I assume you have not actually sliced an eye, but what it contains, is your seeing of probably multiple images. You have yourself "distilled" these into your drawing. I believe this is a very valuable skill. Your drawing is a "mashup" of all eye schematics you have seen :) I would say that the value of your drawings, compared to pictures or a jumble of different kinds of illustrations, are immensely larger. I also think that it is important to keep up the skill of pencil and paper.
"But I want it digital..:"
Yes, I fully understand that. I have had tablets for many years, I am very fond of them. But it will never be the same as pencil and paper. They can complement each other, they can maybe enhance each other, but they are fundamentally two different tools, and therefore two different processes. Pencil and paper are an extremely tactile thing; something the tablet do not have. Variety of the paper, the sound, the pressure, the actual ink flow or pencil hardness etc etc.
What I think you need to do, is to do some mental planning, ask some hard questions.
Some illustrations and drawing you can do directly with the tablet. These will, most likely be good, but lack that elegance that graphite will give. You will loose some life, the magic, artistic "it". You will of course get better and better, but start out by asking the question:
So, what images are essential to have as vector?
Really. Maybe these should be done directly with the tablet, maybe just with some simple guiding image behind. You will sacrifice some of the elegance, spontaneity and sophistication of the trad media drawing. Or, draw it with ink. Ink is pretty straightforward to trace, particularly if you keep the edges pretty sharp.
An example of this at the bottom of this post.
Pencil and ball-point pens
To make a pencil sketch into vector is a bit of a nightmare. You need to scan it, go through Photoshop for contrast, sharpening and whatnot. Then into Illustrator and fight with tracing. I have come to the conclusion: I only do it when absolutely necessary. I have landed on the compromise:
If I want the energy, artistry and "credibility" that goes with physical media, I will sacrifice vector. Sometimes: I will draw over the images, and create simplified vector images. I also know, that if at some point I really want vector I could, but the vectorising process takes away some of the life of the drawing. That is in the nature of the process, and I think the value of the hand-drawn in most cases wins hands down.
I know this did not directly answered your questions, but I hope it could be a contribution nevertheless.