What you describe is still an observational study, just a very expensive one now. It's still quite possible you'd fail to measure something important you hadn't

What you describe is still an observational study, just a very expensive one now. It's still quite possible you'd fail to measure something important you hadn't thought of. Better to spend some of that budget instead on a randomised controlled trial but there are likely issues with getting people to follow their prescribed diets to the letter