Don’t be discouraged, conversion tonecurves will be nowhere near the curve shown in the lower screenshot. I found that the plano is easily good enough as a backlight.
If you want something better, you might want to use a white surface lit by the sun or a couple of tungsten lamps in a setup lika a copy stand and put a few feet between the negative and the lit surface. Not very practical, but best CRI you can get…
I’ve also just checked the plano for how the LEDs are mounted. I thought they were spread over the surface, but it rather looks like edge-lighting… See the photo taken from the panel’s backside:
Looks like the diamond shape is caused by the light-guide.
Here’s a shot of a negative treated with the settings that revealed the diamonds:
I found not the slightest hints of the diamonds, no matter what I did. This extreme treatment brought out the ribbing of the sofa’s covering - and imaged in the medium format negative shot against the light - and it would take a heck of a lot of postprocessing to make it visible in a real image. BTW, I keep my negatives an inch off the surface of the backlight.
Summary: Stay cool, the plano will do its job, as will many other products or DIY solutions.