I think pretty soft, white sheers, pinch pleat on rings would be nice. Sort of like this, only from the ceiling down.

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Since you have a heating unit along your floor, I think you best window covering would be a fabric roman shade.

Unless you remove the existing treatment, you have no space at the top near the ceiling. Rings won't work, because the rod would have to put placed high and the rings would be higher, and then the top of the frame will interfere. I would take down the existing blind and replace with a up or down Roman Shade in whatever fabric you want to light control you need.