What should be my first upgrade?
As everyone above points out, you'll pay double or triple to upgrade piecemeal.  Save up buy a whole bike.  Used if you have to.

The two items that I would consider are NOT on your list:


If you are at the upper limit of your seat height, you might consider getting a longer seat post.  It's hard to pedal if you're not getting good leg extension.  (The frame is probably also too short for your torso+arms, but the legs matter more.)
Clipless pedals and shoes.  Your feet will thank you.  Plus, you can carry them forward to your next bike.


You can also upgrade tires once these wear out.