Okay great, that helps a lot, and I think you're generally on a good track. :)
I'd suggest adding a step to your underwork -- and this should be the very first thing you do, before you even put the major shape bubbles in place. Decide on and record with a single pencil stroke the "action line/line of action" Here's some examples of what that means with human poses, but it's the same concept with animals. https://line-of-action.com/gesture-basics-1-line-of-action/ Then put your shapes on top of that line of action, letting the curve of the line influence where your major shapes go. This can help to unstiffen your poses considerably.
To get you into this concept, I'd also suggest that you do some warmups with much much shorter poses, max 2 minutes but I'd encourage you to even try 30 seconds or less, with your focus primarily (maybe only!) on capturing that action line, following this practice philosophy: https://line-of-action.com/happens-30-seconds-isnt-enough/ before you try your 30 minute poses again. (The class modes on this site are built for exactly this kind of practice, if you want to take advantage of them!)
A lot of times stiffness is an indicator of concern about getting your drawing "wrong," whereas the impression of fluid motion comes from, well, allowing more fluidity in your technique.
Hope this helps! Definitely show us your next batch of practice sketches!
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