Maybe I’m just picky!
Aperture is important. Using an aperture 2-3 stops down from wide open maximizes lens performance on most macro lenses, avoids contrast reduction from diffraction, AND avoids deep depth of field that would render any pattern in the light source such as the surface texture (or fingerprints and dirt!) on a diffuser.
SPACE between the film and the diffuser is also important. The light diffuser should be several inches behind the film, to ensure that it is out of focus.
Light sources that do not cover the full, continuous spectrum of visible light are likely to result in color shifts that you do not want. That’s why nearly all of the 78 optical color negative printers in the lab I worked for in the 1990s used quartz-halogen lamps, projected through translucent dichroic color correction filters and then very heavy Plexiglas diffusers (Brits call it Perspex). Tungsten filaments output a continuous spectrum — but it’s lop-sided towards the red. That’s why film has an orange mask, to compensate for it. Remember, you’re working with opposite colors… To add blue to a digital capture or a print (i.e.; compensate for a LACK of blue in the light source), you take it away from the light going through the negative.
Electronic flash is a discontinuous spectrum light source. It does a REASONABLE job of rendering color, but what it leaves out is noticeable in side by side comparisons. There is another thread here in the forum that compares a lot of different light sources, with examples. It’s worth a read.
YES, the diffusion is very important. So is evenness of illumination. The light intensity has to be consistent from corner to corner, edge to edge. Absolutely no patterns or details should be visible in a capture of an unexposed, processed film chip, other than surface dust and film grain. If you see fall-off, or areas of the frame that are darker/brighter than others, make adjustments.
Blocking stray light from causing lens flare or reflecting off of film or film holders is also very important. Everything surrounding the film, including the film holder, should be black. Back in the 1980s, when I duplicated thousands of slides as an AV producer, I wore a black apron. I worked in a dimly-lit copy room with black walls, a black ceiling, and a black shroud mounted to the macro lens to block reflections from the camera body. We never had issues with flare, because we eliminated the sources.
In-camera custom white balancing off the light source is not necessary, because you should be recording only raw files for use with NLP, AND because you’re always photographing through a colored film base. The procedure for white balancing with the eye dropper tool through the film base between frames takes care of it most accurately. Every color film emulsion has a slightly different color mask, that varies with brand, film type, film speed, emulsion batch, age, negative storage conditions, and film process accuracy. So the eyedropper tool eliminates all those variables in one click.
While this may seem like over-thinking, I assure you, it’s knowledge gained from over three decades of experience in the photo lab business, and 52 years of serious photography.