Roman shades and a more interesting bedspread will go a long way. I would not built in a wardrobe --- if you ever have an injury or disability, you'll need the room. You can often get very nice vintage armoires for reasonable prices at antique stores. If you like your bedspread, you can get a throw to add some color. Or with pillow cases.

While I like jolts of bright color, you can pick up one of the muted colors in your rug --- any constrast will add interest.

Wanted to say ‘more clothes storage’ not closed storage.

This armoire is very sweet looking with the scalloped edges:

https://www.westelm.com/products/kids-josephine-wardrobe-d810/

I’m a fan of top down/bottom up shades so you can get natural light but privacy during the day by having the top half open.

I actually wouldn’t add much to the window wall, especially since you like the existing trim. In a 10x12 room, keeping that area visually light will help the bedroom feel less crowded.

Instead, I’d make the bed the main focal point — a more substantial headboard, layered bedding and one muted accent color would give the room much more character without taking up any additional floor space. I also think the built-in wardrobe where the dresser is now could work really well. Taking it close to the ceiling would give you much more storage while visually simplifying that side of the room.

I recently put together three very different bedroom transformations that show how much you can change a bedroom through color, texture, lighting and the focal point without relying on a completely different layout. There may be a few ideas you can borrow for yours:
https://youtu.be/nEcPGozCbFY