Not sure if this helps - and I'm just starting to get the "itch" to design my own model....but after building the Star Destroyer (75252), the Republic Gunship (75309), and the TIE Fighter (retired - 75095) - in terms of "long flat surfaces" - 3 to 4 plates thickness with interweaving longer/est plates for coverage tend to be the trick.
Especially if you have no struts or frames to draw support from. The only thing I've noticed is they tend to warp slightly under their own weight - but can be resolved with extra structure or framing from any number of areas.
My point being - your on the right track - the current iteration looks very good - better over your last one.
The TIE Fighter was a good one to think of in this case as its got very long panels - thickest I've seen yet - yet its rigid as can be.
Weight is a concern however. The Star Destroyer is very tenuous in that regard - maybe look to the TIE Fighter Panels for inspiration or Gunship Wings - granted they aren't holding much but could help.
Again - noob here - so feel free to disregard.