Greenscreen recommendations
Nice explanation of your problem.
I would simply move the two softboxes higher.

To minimize the falloff put the lights about the center of the framing of the camera. If your camera is targeting the green fabric at 1mt (I see the camera is pointing down a bit) put the lights at 1m.
Probably the softboxes have less power than the umbrellas. Try to add a stronger light on them. Just watch they do not overheat. Safety first.

I do not know if this would help, but If you do not use the window I would put the green screen there because it seems the room is longer in that direction, which will help you have a less green spill on you, and use a longer focal length.