First of all, I must say the design looks great and I’m looking forward to seeing it built!
One thing that caught my eye is that you seem to want to use the 16mm lens for the Pi HQ Camera. I haven’t actually tried that lens for film scanning purposes, but I think the preferred method is to use an enlarger lens with extension tubes to achieve the kind of magnification we need. Myself and others have shown some of their lens setups in this thread. Obviously, this needs a bit more space and probably also weighs a little more, but it works very well and the results look great. I think your design is also well suited to accommodating such a lens setup.