The primary reason for window treatments would be for privacy at night. Any treatment would be opened during the daytime. This is our family room and we spend a lot of time in there. It is the back of the house, but our neighbors are not far and with the lights on, you would be able to see us walking around, watching tv, etc. The other reason would be to add some color or design to an empty wall. There's no room to hang anything decorative except for using the door and window as a base for design.

Originally I thought Roman shades, but when I read "lightweight, whimsical sheers", I thought of Urban Outfitters, https://www.urbanoutfitters.com/shop/draped-shade-curtain?category=window-curtains-panels&color=004 and because all of their curtains are on the more whimsical side, I think you could pick any to use with a tension rod, because you don't have room for proper curtain rods, https://www.urbanoutfitters.com/window-curtains-panels

What words don’t make sense to you? Maybe this would help..... On the left side of your photo you have three windows. You would hang one drape panel on the left side of the first window, one drape panel on the left side of the second window, one drape panel on the right side of the second window, and one panel on the right side of the right widow. Then you would use a faux (“fake”) roman shade on each of the other windows on each side of the fireplace. The roman shade is shown in the first pic decoenthusiaste posted. Faux means that it is there for looks only and doesn’t raise and lower to cover up the window.

The height of your panels works for the room, leaving the arched top window to be a view to the sky. Is there a reason to go higher? if so you can do an arched to set of panels or top treatment. Please keep in mind that you have at least 1 more window in that spacey and if you do a higher treatment on that window it may bring down the height of the room and be unbalanced with the other window(s). I agree that shades inside mounted would look good if you need light blockage, and I prefer to have panels out further to each side exposing more of the glass. .....Did that help any?