Leak-proofing aluminium windows in a minimalist house
Everyone wants their house to be leak proof. No one gets it. I live in Florida and am always getting calls to re-caulk windows in high rise buildings that leak. That's just what water does. You mentioned aluminum windows. They expand and contract with the weather, that will break a seal and water will find a way to get in. Zero or less maintenance doesn't exist. Your 6" concrete upside down "U"  would probably end up pulling away from the wall a bit and leaking there, needing caulk. Since I mentioned caulk, it have improved much over the last few decades. The quality silicone caulk available today does  great job at remaining flexible and while it might not last a lifetime, I've installed some that still great after 30 years. Good luck.