Stiffness mostly relates to shape and thickness. However, changing laminate schedule can help somewhat.
As for first infusing under film: Yes, that works, but you will get a more or less different result, depending on setup.
I suggest for your tests, use the same LRTM mat (do you use rovicore?) and use a low vacuum (-0,6 bar or so) or you will compress the LRTM mat.
Now you can decide which glass you will be using, to get a desired thickness / stiffness. Also keep in mind that the part might not be stiff, but once mounted it most probably will hold.
For the top mould get one of your best laminators, and get him to add CSM450 layers up to the desired thickness. Every layer will give you 1mm of thickness. When done with care thickness variations should be minor. This will save you from ordering wax, but will add labour for getting the surface right.
Also keep your molds in a high polished state, to improve longevity. Let the guys that do the demoulding, also do the waxing. (preferably semi perm). These are the guys that know the difficult spots, and can tell when a new layer of wax or semi perm is needed.