> And I'm afraid that last 0.1% of aquisition is pretty much impossible to obtain deliberately unless you abandon your native language. Even your English, whilst technicaly good, has a couple of flags

But those last .1 % are not very useful anyway if you're not living abroad. You want to be able to communicate fluently and not make noticeable mistakes, but it's not necessary to seem like a native speaker. Unless you want to become a spy maybe.