to reduce stiffness, draw the ribs and hips separately. i would recommend using ovals for this because straight lines are less effective at making the pose relaxed.
with this id also use a curved line to mark the shoulders.
i will also say that limbs aren’t completely straight either, look closely at them and try and recognise what parts are thicker than others as this is what creates the organic shapes (eg calf muscle at back and quads at front tend to give legs more of an s shape)
to paraphrase, don't use as many straight lines 👍
