30485pb01 Windscreen 8 x 6 x 3 Curved Top Angled Canopy Pattern, how to finish?

[Philippe Hurbain]

and is also bent in a way that slicerpro can't handle (in a satifying way)

Perfectly right... you can't properly do that in one step, since some surfaces are parallel to projection direction!. You have to do a partial pattern viewed and projected from top, and another viewed and projected from side.
In the attached file, the pink area will get the top projected pattern, yellow area will get left projected pattern, and green area will complete the half windscreen pattern subpart (or, since here the pattern is simple, put directly the pattern and its x-mirror in the patterned part). The only tricky part is to have both patterns meet properly (here it should be easy).

most of the printed area shown above uses primitives

Either inline everything in the former files, or check "expand subfiles in former (-sf)" option. In this case, don't forget to specify your LDraw folder in search path.