With that size room why are you concerned with hiding the speakers? You are already going to have 2 huge subs out in the open. You will get much better sound with free standing LCR speakers at a lower cost. I had a similar situation as you in my old house and built a dedicated theater room with stage and riser. Tower LR and full center. Wall mounted surrounds. New house, smaller room but still have the full LR tower, full center, wall mounted surround - but added 4 in-ceiling speakers. Sounds and looks great as a 5.1.4 system.
I certainly understand and respect the clean and simple, just ensure you understand the cost (and not just $$).