I’m using RPi4. I believe @Manuel_Angel is using RPi5 now.
I downloaded the file that you posted. Using Rawtherapee only changed the exposure and black sliders.
As you can see there is some very visible horizontal banding…
I see that you have replaced the bulb, and now are using the LED strip. I believe it is a white LED only light… can you confirm?
If that is the case, consider using a regulator or a better power supply, and that should eliminate the light banding.
In your comments about the reproduction of the red banding due to black clipping:
You were right.
To have a perfect flat light is very difficult, so the biggest concern on the above is the banding, and the large square patch on the lower left area.
For comparison, here is what I am getting.
I would also be concerned by all the light hitting the lens (probably the cause of the square patch). No need for 3D printer, one can use a black heavy stock paper (or flock paper if you have it) and build a cylinder to reduce the undesired light.
Here is an example of what I used on my first scanner version.
Lastly, please keep in mind that the higher the red gain, the more visible the problem will be.