Hi there!! Your understanding of anatomy is really strong because of this method! All the figures look realistic to a human body.
Usually, what helps me with stiff forms is doing 1 minute gestural drawings. It forces you to focus on the overall form rather than the anatomical elements. Doing sessions of a handful of 1 minute drawings might be a good method to try out!
You're doing pretty good in terms of mannequinization and that's helping a lot with keeping proportions alltogether! Althou, gesture is more about flowing with the volumes and how them interact with eachother. You're halfway there though, proportions are awesome, you could keep a little more of the focus on line, as simple as it can be: C, S or I. And remember, is about flow within volumes, not copying contours. Your work is awesome!
It sounds like the new approach you are looking for is exaggeration. Look at what you are already doing, contrast it in your mind with somebody standing perfectly straight, and then test out how far you can go in the opposite direction, how extreme you can tilt the masses and twist the joints, before the pose really starts to break apart and look goofy.
A good measure would be, that about half of your drawings indeed should start to look goofy, as that indicates that you are actually max-testing and pushing the limit.
And then look at the half that does not look goofy and marvel about what is possible. That way your stuff will definitely stop feeling "stiff".
Don't aim at recreating the reference, you are better then a meat-based photo filter. People looking at your pics will never rate them by how closely you stayed with the reference, but how easily they could read what is going on. You are skilled and disciplined enough to do the photo filter job, you don't have so much left to proof by repeating it, now push beyond it.