IMO what's important for any study/exercise, is the ability to observe well and remember it later. Make observation an active effort, rather than passive.
You could try doing purely analog work for the next while, which can help (if historically digital-focused artist) a person to not solely rely on tools like "undo" and "layers" for a bit.
Study from "life" / life drawing
Use reference to do fast gestures/line of action drawings/contour/shape/value/expression/etc. studies. I think it's easier to focus on one aspect at a time. Some websites have automated timers built in, with with others, you can set your own timer IRL.
https://www.lovelifedrawing.com has a basis for a curriculum, and then you can use any website with visuals as the subject to be drawn.
Sites with timers:
https://pose-trainer.com/en/
http://reference.sketchdaily.net/
https://www.posemaniacs.com/ (currently down, but I've used it in the past)
Reference (mostly for human figure)
Just a note that it's good to seek out diversity in your reference material, as a lot of stock photos can lean towards very fit/idealistic body types, race presentation, etc.)
https://www.fatphotoref.com (need to apply to get access)
https://www.bodiesinmotion.photo (moving bodies)
http://referenceangle.com/ (faces/heads/expressions)
https://www.artstation.com/artwork/6a0Par (female head lighting)
https://www.artstation.com/artwork/GX3Ax1 (male head lighting)
https://freephotomuscle.com/ (muscle men doing "everyday stuff"; this website is hilarious)
https://ecorche.anatomy4sculptors.com/muscle-man
https://nappy.co (black and brown people stock photos)
https://x6ud.github.io/#/ (animal skulls)
(etc.)
Practice inking
If you want to really get to the basics (understanding pen/ink), Dunn has many video exercises.
Then you can practice inking over someone's comic sketch/pencils. It can force you to break out of tendencies when lining/refining your own artwork and focus on keen observation, control, and accuracy.
Movie/film/TV still studies, or Master studies
Good to practice composition/layout/framing/colour/values. Pick your favourite film scene/still (or famous painting/drawing) and replicate it.
Reuben Lara and Aaron Blaise also have good tutorials on various art topics. I've paid for their tutorials before and felt that it was worth it!