The problem with exchanging BTC for untraceable cash or vice versa is the people you would be dealing with are probably criminals who are not above robbing you of your BTC and/or cash.
Iirc there is a bitcoin “atm” somewhere in my city
If the cash loaded into a bitcoin "atm" is legal, it's traceable. If it is illegal then your results may vary, most likely to your financial and legal detriment. If you think a bitcoin "atm" is a solution to this problem then hopefully your lawyer accepts bitcoins.
Every Bitcoin ATM I've seen here require you to look into the front-facing camera and insert/swipe ID.
An easy solution to that is to find a homeless person nearby and use fake IDs -- i.e. bribe someone else do the transaction instead of you