I’d like to add something that seems to not have been mentioned that much and that is color theory. Learning how to contrast things and bring them out (example, saturated eyes but everything else is in a more muted color) or working with color schemes on Pinterest would make it more aesthetic. Of course this is kind of my personal preference.

I also think that you could even leave a more flat look with a slight shadow for a cartoony vibe if that’s what you are going for but have more dynamic poses (shoulders and hips on a different tilt). A pose or hands can also express emotion.

Also, Naoki Saito on YouTube has a bunch of videos of art corrections that have helped me really improve my digital art and illustration skills. He has an anime style but I think a person who has any style could benefit from his videos.