Using a professional light box instead of a tablet should should give you even color.
I have no experience with building camera profiles, so I can't say if that's going to work, but Lightroom isn't designed for working with scanned negatives.
You will probably get much better results with Photoshop - see this thread: Re: Colour Film Negatives
I'm using the method described by K´mo in post #17, which involves using a sample of the film base color on a Fill layer.
The Color negative preset works well with some negatives, but I found the method in post #17 to be generally better.
Use whatever works for you ...