Minimalist improvement on tiny shower?
I would check on the type of shower curtain they use in hotels that have an arced curtain rod. This would give a slightly more room.

The reason the shower curtain sucks in when you turn the water on probably has to do with hot air rising and pulling in cold air at the bottom of the shower. Try leaving the curtain slightly open at the back of the shower to allow air to enter the shower without pulling the curtain in.

You could also put a layer of opaque material on the window and get rid of the window curtain.

Good luck!