Ok but is it really illegal? I doubt there’s a law on who can send who private BTC transactions? Or is it actually possible to mark this as laundering?
There's nothing illegal here. The goal of privacy is to avoid giving anyone a reason to suspect something. In the case of money laundering specifically, you want to turn money that looks extremely suspicious into money that doesn't look suspicious at all.
Little tiny 'gotchas' aren't going to save you from a determined investigator. They are trying to follow a trail of evidence so they can produce more evidence. What you need is a clean break, so that the investigator hits a dead end and has no productive leads they can follow.