I've been doing something similar. I started with a 3D printer approach, then two cheap aliexpress C-beam linear actuators and finally managed to acquire a 2-axis microscope stage for cheap. The key I have found is that any issues with alignment can actually be solved with focus-stitching.
The real problem with most scanning setups is actually getting accurate color out of color negatives. The common wisdom these days is to use high-CRI light, but I believe that approach is flawed. Film scanning is not an imaging challenge, but a rather a densitometric one. You don't actually want to take a photo of the negative in a broad spectrum because the dyes in photo negatives were never intended to be used in a broad-spectrum context. You actually need to sample the density of the dye layers at very specific wavelengths determined by a densitometric standard (status M) that was designed specifically for color negative film. Doing this with a standard digital camera with a bayer sensor is... non trivial and requires characterizing the sensor response in a variety of ways.
Basically the hardware is easy, the software is hard.
