One thing I would suggest, as someone else who is on a mission to master hands, is to try drawing a few from imagination as well as observation. I've found that forces me to think about the structure of the hand rather than just transposing what's on the screen onto paper, and also helps to identify which hand positions and gestures I'm starting to get the hang of, and which need more work. When you then go back to drawing from observation you'll be able to focus in and fill in the gaps in your knowledge.
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