it doesn't make you achieve 100% privacy, DEX and P2P platform only help you to maintain your privacy, not hide it from the government and chainalysis.
It is impossible to hide from the government if they are particularly or specifically looking for you, people maintain their privacy so governments, chainalysis and other people would not be able to trace them, but i think we all know that if the government go on a manhunt for you they are going to find you; but rarely do people have their government looking for them, so maintaining privacy is still very important.
Is there an actual, reliable way to exchange bitcoin for cash in hand? My best guess would be a P2P transaction in person, but that's quite unlikely.
You need to know someone in person who you trust, you can exchange your BTC with the person, but you must look at the disadvantages and risks of selling BTC that way, if you are satisfied, then you can go ahead with it.