I think your self-assessment is accurate that the longer drawings feel a bit stiff. Giving the spines a bit more of an S curve and letting that show through the whole torso like the 2nd from the left of your 30 second drawings might help it seems to me.
Stylistically very cool and I think it's impressive how you managed to get the foreshortening looking convincing with this style. There's not much room to grow while use this particular style though, as it doesn't really allow for fine details or realistic proportions. A focus on refining details and more intentional lines seems like the next step in my very humble opinion.
I'm also getting the prompt to request access
