Question regarding inductor peak voltage
This does not answer your question, it's more of a comment with a picture.
I don't get nearly as much gain at 20 kHz as you do, but try to get the input waveform as close as the one you have in real life. It could also be something in your LED setup if it's not entirely linear.
40 by 43 cm is a large panel considering the 20 mA peak current limit in your inductor, but your irradiation might be low as well.