Been a few weeks, not that I haven't worked on this, but my progress is more in the finer detail phase now, though I still have made adjustments to overall face structure, and expression, and I keep adjusting the eyes. But I was waiting to post until I had a little more progress.
I started refining the hair, though I still haven't decided exactly on how it will end up. I may simplify it more, but here's where it stand now:
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I also did a couple more subdivisions of just the lips and around the eyes, so that I could add more detail to those areas. It seemed that when I applied another subdivision to the entire head, it caused Blender to slow down due to the extra detail, and I couldn't work very well - the cursor would lag behind so I couldn't draw in any detailed way. I'm not sure if it was only a temporary problem or it's just due to my older laptop not being powerful enough to handle it.
Since the lip area now has more vertices and faces, I was able to refine and add some more lip texture. I looked at tutorials on using alphas to add these details, but I just decided to sculpt them in using the normal Blender brushes instead. I mostly used the Draw Sharp and Draw brushes at low pixel sizes and medium strengths. I also used the Fill brush to flatten out the details, which looked good close up, but further away were a little too deep. It made the texture more subtle. I still may tweak them as I go. Here's two pics from the same angle, first with standard "solid" shading, then with rendered lighting.
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I'm not sure how necessary the extra lip detail is, given I my original plan to 3D print this as a mini bust around 6" in height or so. There's no way my printer will be able to show that level of detail at that size. But I plan on subdividing the eyebrows as well, as they look a little too pixelated right now. Plus, the more detailed I make it, the more I learn, and it never hurts to have more detail in case I want to print it larger or use it just as a digital render.