A Roman shade would work well. But first, I'd try moving the curtain rod up near the ceiling to see if that improves the look.

I would go with the roller blind and no curtains.

The problem is how busy it looks. One idea is to look for a fabric (and lining if thin) that suits the room’s decor. Simple, elegant and light control options.

Another idea is to keep your blind (pull down a bit) and fill the space above window with a stationary valance.

If still wanting curtains, raise rod (I’d suggest less heavy), ending at window sill and not tied back.

If the view is not a priority, wood shutters would suit your window’s framing and light will be easy to control.

I like the shutter idea. you will be able to block the light but still open the window when you need to.

What room is this window in? a bedroom, hallway?

I would paint a double hung set of shutters (off the rack in a white factory finish)

and paint everything white, window trim and all.

It will compliment the white radiator and won't be an eye sore.

Roller shade or roman blind. Or a bottoms-up shade for privacy/airflow with the upper window open (when you don't mind the light).

A roman blind hung high , what is the space that you need to block light ?

I normally don’t like Roman shades but this window is begging for one.

TDBU top down bottom up blind. The bottom half can be shaded more often. Lightweight cellular honeycomb is the most common version.