Most of my experience drawing digitally was for making my own decals for plastic models using Corel Paint and Corel Draw, Very basic stuff, logos, national insignia etc. Nothing as intricate as your eye, although the basic principles apply. Using a mouse or trackball I found it much easier to do line work with the point and line tools* rather than freehand as if I were drawing by hand. I could never get good line quality drawing freehand so only did it for very small details and connecting lines. *By point and line I mean where you choose two or more points and then a line snaps in place. Also including the curve tools, where you again choose two or more points and then drag the line until it makes the curve you want. The curve tools require some trial and error if you have not used them, but they can be surprisingly controllable once you get the hang of it. Unrelated, but I've been pleasantly surprised to find how much drawing and photography overlap. Different techniques but I find much I learn about one bleeds over to improve the other when it comes to composition, light and value.

Wow, drawing with a trackpad is definitely an added challenge! As much as you can, trying to use longer, confident strokes can help reduce some of the scratchiness of the lines. I see you have a lower opacity sketch, and that's great, I think that'll help a lot with checking proportions and refining them on a new layer. I'll link a video on eye anatomy that might have some value to you. Best of luck! https://www.proko.com/course-lesson/how-to-draw-eyes-anatomy-and-structure