Copying color negatives with camera

Thanks for taking the time to post this.  I've been looking into doing this myself, and had planned nearly the same approach.  My plan is to make a platform to hold the camera (Sony a6000) and lens (telephoto zoom with macro adapter -- at ~6" I can fill the frame).  On the opposite side will be a flash (Yongnuo 560 III, operated by rf remote) shining through a diffuser plate where the negative will be mounted.

Using a zoom lens means I don't have to build in any rails, and I can easily turn this platform into a box with lid if I get tired of turning out the lights.

I'm especially interested in (troubled by) your experience with the white balance, though.  I also planned to try the camera calibration of white point using undeveloped film approach, at least as one option.  Otherwise, I figured I'd shoot in RAW, and figure out how to tweak WB in post.  But I'm not big into PP, and don't have any decent software (LR, etc.).

I do use GIMP, though, but only for minor tweaks on jpegs.  For tweaking WB on a RAW photo, I'd used Sony's free software.  Maybe GIMP doesn't have the latitude needed to make such a large adjustment to WB(?).

My understanding of WB adjustment is that color temperature is only a part of it.  Color temperature just describes a line on the CIE color chart that corresponds to black body radiation.  But, if you're shining your light through negative film stock (essentially a color filter), you're departing from this line.  On the Sony a6000, you can adjust white balance by color temperature, but then you can also make color corrections beyond that, which I'm assuming I'd need to do.  Is this an option on your camera?  It still seems to me like the proper WB adjustment will do the trick, but until I try it, I won't know the limitations.

I'll be very interested to hear of your progress, and thanks again for posting!