One thing that might be helpful is to import an orchestral mix as a track and then compare it to whatever you’re working on.
The music itself doesn’t matter, what you’re doing is seeing how the layout you use compares to what you are hearing in the mixed imported track from World Class orchestra of choice.
I don’t mix that kind of music, per se. I do find that if I put up some sounds (patches) that are intended as “ensembles” or “sections” I’m automatically in the right playing field as far as the overall sonic space sets up.
The dedicated (expensive) orchestral synth instrument packages seem to have some excellent set-ups for achieving the orchestral sound and mix baselines.
Watch some of the top-flight shows like Great Performances on PBS and make notes about how you see things mic’ed. What you’ll see is that each section is mic’ed and some of the soloists or section leaders also have individual microphones. The sound field for an orchestra is sensitive to a 3-D sound space. The bass section is behind the cellos, the Violins are up front, stage right, the horns are in the back rows (drinking ), percussion section in the back and on risers. Orchestras play in concert halls and the great halls impart their own sound to the music played in them.
So try to mix for the “Stage” your Orchestra is on from your ideal “seat” in the “hall.”
Midi Comps are used in lots of TV shows and ads when a real orchestra is too expensive and time consuming.
Sound on Sound magazine most likely has some useful articles about this.
Have fun and good luck with your mixes.