For security a simple fix is putting a piece of wood on the bottom of the door so it can’t be opened. A better fix is having a secondary lock installed.

You might really enjoy the deck access so blocking the door seems short sighted. Will a desk fit on the wall with the entry to the room or the end of the wall with tie slider?

If bed would fit on walll next to sliding door, I’d put desk on the long wall instead so you can benefit from the natural light while working and the ability to add some storage.

The sliding door will never look like a window, but decorative film will make it more appealing and provide privacy. You can also get thicker curtains vs your sheers to pull across at night.