Keep up the good work! I can see progress already just between some of the images in the folder!
Some points I'd focus on are your proportions, and your shading.
Honestly, proportions will mostly come with practice. Critiquing yourself after a sketch session, or taking note of parts you think look odd can also help.
The justification for most of your shading is pretty good, but it can be a bit heavy and distracting sometimes, especially in your pen drawings.
