Hello! These are pretty good gesture drawings for only two weeks of work! When it comes to improving art skills, the main thing is practice, and a lot of it.
One thing you can do is study some proportions a bit. While gesture drawing is basically throws them out the window, it can be prretty useful to have a general sense of where things are, or how long other parts are supposed to be. The head seems a bit small in some of the drawings. Some quick proportion tips can easily be found online. There are some pretty cool things about our proportions that can really help when drawing. For example, people are (usually) about 7 and a half heads tall. Generally, about two head up from the bottom of the feet, you have the knees, another two up from that is around the top of the pelvis. Half a head up from there is the naval. Another head up from there is the nipples. And there is about a head length between the nipples and the bottom of the head. If arms are resting at the sides, the elbows are very close in height to the naval. Also the shoulders (for a male figure at least, I don't know for female) are usually about two head lengths apart. I think that the foot is also about a head length long from heal to toe. (Theres probably other things I'm forgetting, though)
Don't get too caught up in this for gesture drawing, perhaps do some drawing studies separatly where you focus on anatomy. The two sort of go hand in hand. You need a bit of anatomy for the gesture drawings to keep them looking human, but more complete drawings without gesture can become really stiff.
If you want things to be more fluid, one thing I like to do is use a bit more flowy curved lines for the arms and legs (kind of like how you are doing the spine). It can really give the limbs a bit more fluidity if thats what you're looking for (this could perhaps be the next step once you get more comfotable with the anamomy, and you can switch from the more skeletal limbs to this method)
These sketches look great. The 30 second ones are particularly impressive for the time span!
One thing you could perhaps do for your longer poses is to really use the full range of possible values. Don't be afraid to really use some darker shades for the darkest shadows! It can really help add depth and make drawings pop a bit more.
If you feel like you've plateaued, perhaps switch mediums? For longer sketches perhaps try toned paper with charcoal and white pastel pencil (for lights). Or maybe try ink.
