This class was more of aligning and being precise, which always is a challenge for me.
When I really zoom in on the brush I see that somehow the strokes of half circles don't add up in the final brush. Not really sure how to avoid that. I had "Align dashes to corners and path ends" checked in my strokes panel.
Apart from that, I got the brush finished.
Here is the workspace on my project. Added variety to stroke weights, but I guess it's better to keep it consistent (for next time).