Are there any shades on the windows? Hopefully there are and you don't face west! I would worry about fading without some type of solar shades. You don't want to have to close curtains to control the sun and then block your views.

Are you after window treatments to control sunlight and fading, for privacy, or for more visual interest or softening texture in your space, or a combination of some of these?

Befriend your neighbors and learn what kind of shades they use so you don't have to reinvent the wheel or start your research from scratch. I live in a building where neighbors show each other our blinds and shades because we all live with the same window constraints.

Show us another picture that includes a view from the top of the window to where the walls meet the ceiling. Your wall and window constructions and materials are going to dictate a lot about what you can install here and how.

https://www.shadesbydesign.com/blog/floor-to-ceiling/

Go with a light oatmeal or off-white linen. It'll keep the room from feeling too dark with that big black sectional and balance out the grey rug. I'd hang them high and wide to make the windows look even bigger.