Hi! I realize this comes extremely late, sorry! I had not been active on the forum lately.
For most of the rendering issues that are currently open, some knowledge of OpenGL and vispy will be necessary. Other than the issues that are currently open or the features that are missing, I can’t think of a “coding challenge” that might be helpful.
To understand the rendering code, I would suggest using core/primitives.py as a starting point and then looking at sketch/renderer2d.py and sketch/renderer3d.py.