I think your off to a great start in your thinking. A flannel sheet could be tacked to the wall for your design wall or to a roller blind. Your "study" desk could double as your sewing machine table. The price of new card table is high, I would opt for a used desk, it will be much sturdier and you wont get the vibration from you machine. If you can find a decent, usable one cheaper than a card table. Local thrift shop is a good idea. For quite along time my sewing machine sat on a peice of plywood between two old small filing cabinets(free) It worked, I covered it with a plastic table cloth from the dollar tree for you guessed it a dollar. Anything is possible just use your imagination and watch the trash in your neighborhood, one mans junk is anothers treasure!
Good luck with your room, and your studies!