I personally think that low, platform beds look great. So, if that is something that you are open to I say do it! Crate and Barrel, and RH are going to charge you a lot for very little visual in a platform bed. I would look for cheaper solid wood construction. Last I remember Amazon had some good options.

Can you install a few skylights? Also Wall sconces, one on each side of the window, and a large bright bold area rug.

If you want a headboard, I would put the bed on one of the low sidewalls. If you put it under the window with a headboard, you'll just be fighting the architecture of the room. That never works.

I would also paint the entire room, walls and ceiling and molding, the same color.

I would put the bed on one of the low sidewalls.

I would put the head of the bed on the window wall and have drapery panels extend across the entire length of the highest ceiling so the window looks wider.